Gapo4 Single Crystals: Growth Condition by Hydrothermal Reﬂuxing Method

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Article GaPO4 Single Crystals: Growth Condition by Hydrothermal Reﬂuxing Method

Denis Balitsky 1,*, Etienne Philippot 2, Vladimir Balitsky 3, Ludmila Balitskaya 3, Tatiana Setkova 3, Tatiana Bublikova 3 and Philippe Papet 2

1 Cristal Laser SAS, 54850 Messein, France 2 ICGM, CNRS-UM-ENSCM, UMR 5253, Université Montpellier, Place Eugène Bataillon, CEDEX 5, 34095 Montpellier, France; [email protected] (E.P.); [email protected] (P.P.) 3 D.S. Korzhinskii Institute of Experimental Mineralogy of Russian Academy of Sciences (IEM RAS), 142432 Chernogolovka, Russia; [email protected] (V.B.); [email protected] (L.B.); [email protected] (T.S.); [email protected] (T.B.) * Correspondence: [email protected]

Academic Editors: Nikolay I. Leonyuk and Victor V. Mal’tsev Received: 27 August 2020; Accepted: 30 September 2020; Published: 2 October 2020

Abstract: Bulk GaPO4 is an advanced piezoelectric material operating under high temperatures according to the α-β phase transition at 970 ◦C. This work presents the technological development of a hydrothermal refluxing method first applied for GaPO4 single crystal growth. Crystals of 10–20 g were grown in mixtures of aqueous solutions of low- and high-vapor-pressure acids (H3PO4/HCl) at 180–240 ◦C (10–20 bars). The principal feature of the refluxing method is a spatial separation of crystal growth and nutrient dissolution zones. This leads to a constant mass transportation of the dissolved nutrient, even for materials with retrograde solubility. Mass transport is carried out by dissolution of GaPO4 nutrient in a dropping flow of condensed low-vapor-pressure solvent. This method allows an exact saturation at temperature of equilibrium and avoids spontaneous crystallization as well loss of seeds. Grown crystals have a moderate OH− content and reasonable structural uniformity. Moreover, the hydrothermal refluxing method allows a fine defining of GaPO4 concentration in aqueous solutions of H3PO4,H2SO4, HCl and their mixtures at set T–P parameters (T < 250 ◦C, p = 10–30 bars). The proposed method is relatively simple to use, highly reproducible for crystal growth of GaPO4 and probably could applied to other compounds with retrograde solubility.

Keywords: piezoelectric crystals; gallium orthophosphate; crystal growth; hydrothermal method

1. Introduction The development of modern radio communication devices and different kinds of electronic measurement technologies puts increasing demands on the requirements of piezoelectric inventory materials. Generally, this means the values of the electromechanical constants and its temperature dependencies. The most widely used quartz crystals have an α–β phase transformation at 573 ◦C[1]. Thus, application of quartz piezoelectric devices is limited to approximately 300–350 ◦C[2]. There have been many studies investigating new alpha-quartz-like structure piezoelectric materials with homogeneous compositions, low dielectric losses even at high temperatures and significant piezoelectric properties in a wide temperature range exceeding 350 ◦C[3]. The best piezoelectric coefficients of α-quartz-like structure materials have been found for α-GeO2. This compound is stable in the α-quartz structural phase in a temperature range of 1033 to 1080 ◦C. It has a metastable α-quartz structure state at temperatures up to 180 ◦C for crystals grown by hydrothermal method [4,5] and much higher for flux-grown crystals [3,6]. Currently, the use of the flux method

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(top-seeded solution growth (TSSG)) allows crystallization on a seed up to 100 g, but the crystals contain many defects [7]. Another known high-temperature α-quartz-like structure material is GaPO4. The temperature of α-quartzMolecules structural 2020, 25, x stability is nearly 930 ◦C[8,9]. Its piezoelectric coefficients are slightly2 lowerof 14 than for α-GeO2, but the quality and volume of crystals crystallized by the proposed refluxing hydrothermal Another known high‐temperature α‐quartz‐like structure material is GaPO4. The temperature method look both more suitable and reproducible than GaPO4 single crystals [10,11]. of α‐quartz structural stability is nearly 930 °C [8,9]. Its piezoelectric coefficients are slightly lower The existing methods of GaPO4 crystal growth [12–21] are hydrothermal methods, retrograde than for α‐GeO2, but the quality and volume of crystals crystallized by the proposed refluxing temperature gradient “slow temperature rise” up to 250 ◦C methods, direct temperature gradient hydrothermal method look both more suitable and reproducible than GaPO4 single crystals [10,11]. (above 320 ◦C) methods based on retrograde (at T < 250 ◦C) methods and direct (at T > 320 ◦C) fields The existing methods of GaPO4 crystal growth [12–21] are hydrothermal methods, retrograde of GaPOtemperature4 solubility gradient in aqueous“slow temperature solutions, rise” accordingly. up to 250 A°C possibility methods, direct of GaPO temperature4 crystallization gradient under high-temperature(above 320 °C) methods conditions based in on molybdates retrograde (at fluxes T < 250 has °C) also methods been confirmedand direct (at [22 T]. > All320 °C) these fields methods allowof GaPO growing4 solubility GaPO 4incrystals, aqueous but solutions, with a accordingly. series of restrictions. A possibility The of crystals GaPO4 growncrystallization at temperatures under up to 250high◦‐Ctemperature in aqueous conditions H3PO4 or in Hmolybdates3PO4/H2SO fluxes4 solutions has also have been aconfirmed considerable [22]. All OH these− content. methods Crystals grownallow in growing aqueous GaPO H2SO4 crystals,4 solution but havewith a series low OH of −restrictions.content, but The growth crystals rates grown of at {0001} temperatures faces equal to zeroup that to 250 leads °C in to aqueous formation H3PO of4 plate-like or H3PO4/H crystals.2SO4 solutions Additionally, have a considerable the use of hydrothermalOH− content. Crystals methods up grown in aqueous H2SO4 solution have a low OH− content, but growth rates of {0001} faces equal to to 250 ◦C is associated with a number of technique-engineering complexities such as a preliminary saturationzero that ofleads crystal to formation growth of solutions plate‐like and crystals. inconstant Additionally, temperature the use of of hydrothermal equilibrium. methods Crystal up growth to 250 °C is associated with a number of technique‐engineering complexities such as a preliminary by the direct temperature gradient hydrothermal method at 320 ◦C or higher needs very complex saturation of crystal growth solutions and inconstant temperature of equilibrium. Crystal growth by equipment (Pt lined high-pressure autoclaves with an inert gas-injection system) [12,13,17]. The GaPO the direct temperature gradient hydrothermal method at 320 °C or higher needs very complex 4 crystalsequipment grown (Pt from lined fluxes high‐ arepressure very smallautoclaves (millimeters with an size)inert andgas‐injection contains system) impurities [12,13,17]. coming The from the fluxGaPO [22–428 crystals]. grown from fluxes are very small (millimeters size) and contains impurities coming fromUsing the flux the retrograde[22–28]. temperature gradient hydrothermal method at temperatures up to 250 ◦C, it is impossibleUsing the or retrograde very diffi temperaturecult to maintain gradient a hydrothermal given supersaturation method at temperatures of the solution up allto 250 times °C, of the runit andis impossible as a result, or very reproducibility difficult to maintain of such a experiments given supersaturation has a low of level. the solution The slow all times temperature of the rise hydrothermalrun and as a method result, reproducibility is limited by GaPOof such4 solubilityexperiments [8 ,has29]. a The low constant level. The value slow oftemperature GaPO4 solubility rise in hydrothermal method is limited by GaPO4 solubility [8,29]. The constant value of GaPO4 solubility in aqueous acid solutions (H3PO4,H2SO4, HCl and their mixtures) in the temperature range from 200 to aqueous acid solutions (H3PO4, H2SO4, HCl and their mixtures) in the temperature range from 200 to 260 ◦C (Figure1) completely excludes using of this method at temperatures higher than 180 ◦C[30]. 260 °C (Figure 1) completely excludes using of this method at temperatures higher than 180 °C [30].

Figure 1. Solubility of GaPO4 in aqueous solutions (at pressure of 5–20 bar): (a)H3PO4, 1–15 M, Figure 1. Solubility of GaPO4 in aqueous solutions (at pressure of 5–20 bar): (a) H3PO4, 1–15 M, 2–7.5 2–7.5 M, 3–5 M;(b)H2SO4, 1–9 M, 2– 6.5 M;(c) 15 M H3PO4/9 M H2SO4 in vol. proportions (%): M, 3–5 M; (b) H2SO4, 1–9 M, 2– 6.5 M; (c) 15 M H3PO4/9 M H2SO4 in vol. proportions (%): 1–20/80, 2– 1–20/80, 2–50/50; (d) 9 M H3PO4/6 M H2SO4 in vol. proportions (%): 1–20/80, 2–70/30; (e) 6 M HCl [30]. 50/50; (d) 9 M H3PO4/6 M H2SO4 in vol. proportions (%): 1–20/80, 2–70/30; (e) 6 M HCl [30].

A detail study of GaPO4 crystal growth conditions has revealed some optimal parameters for obtaining crystals with a low OH‐ content and reasonable structural homogeneity (low crack density and gas–liquid impurities). The papers [10,18,29,31] describe that low OH− content in GaPO4 crystals

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A detail study of GaPO4 crystal growth conditions has revealed some optimal parameters for obtainingMolecules 2020 crystals, 25, x with a low OH- content and reasonable structural homogeneity (low crack density3 of 14 and gas–liquid impurities). The papers [10,18,29,31] describe that low OH− content in GaPO4 crystals waswas fixed fixed with with the the following following parameters: parameters: temperature temperature higher higher than than 220 220◦ °C,C, solvent solvent of of aqueous aqueous H H33POPO44,, HH22SOSO44 oror a a mixture mixture of theseof these solutions. solutions. Unfortunately, Unfortunately, growing growing bulk GaPO bulk4 crystalsGaPO4 bycrystals known by methods known atmethods such temperatures at such temperatures is difficult is or difficult impossible. or impossible. The principal The problem principal facing problem GaPO facing4 crystal GaPO growth4 crystal at temperaturesgrowth at temperatures up to 230 ◦C up is theto 230 low °C convection is the low of convection the solution, of which the solution, obstructs which mass obstructs transportation. mass Thetransportation. practically constant The practically solubility constant of GaPO solubility4 in acids atof temperaturesGaPO4 in acids of 200–260at temperatures◦C (see Figure of 200–2601)[ 30] °C is the(see reason Figure of 1) unsuccessful [30] is the reason crystal of growth. unsuccessful Nevertheless, crystal growth. it looks moreNevertheless, promising it tolooks use more a hydrothermal promising methodto use a combining, hydrothermal if possible, method relatively combining, high if growthpossible, rates, relatively low density high growth of defects, rates, low low OH density− content of anddefects, reproducible low OH− volumetriccontent and crystallization. reproducible volumetric crystallization. AnalyzingAnalyzing existing existing hydrothermal hydrothermal methods methods of of the the crystal crystal growth, growth, we we havehave foundfound aa hydrothermalhydrothermal refluxingrefluxing method. method. In In particular, particular, this this method method was was applied applied to toα-GeO α‐GeO2 single-crystal2 single‐crystal growth growth [11 [11,32].,32]. It isIt characterizedis characterized by highby high and stableand stable convection convection of the solventof the solvent during crystalduring growth crystal cycle growth and acycle controlled and a solubilitycontrolled of solubility nutrients of in nutrients a quantity in necessary a quantity for necessary the continuous for the continuous crystal growth. crystal growth.

2.2. ResultsResults

2.1. Study of GaPO Concentration in the Solutions 2.1. Study of GaPO44 Concentration in the Solutions The exact values of GaPO solubility are not very important for the refluxing method in terms of The exact values of GaPO44 solubility are not very important for the refluxing method in terms of reproducingreproducing of of crystal crystal growth growth at diatff erentdifferent temperatures. temperatures. In practice, In practice, the procedure the procedure of the crystal of the growth crystal rungrowth described run described before leads before to reproducible leads to reproducible restart conditions restart without conditions any undesirablewithout any spontaneous undesirable crystallizationspontaneous crystallization or seed loss. However, or seed scientific loss. However, interest havescientific moved interest us to measure have moved GaPO 4usconcentration to measure in the solutions. The low filling of autoclave by solution (f < 60%) in the refluxing method allowed us GaPO4 concentration in the solutions. The low filling of autoclave by solution (f < 60%) in the refluxing tomethod use a techniqueallowed us of autoclaveto use a technique reversing toof stop autoclave the dissolution reversing at to a setstop temperature. the dissolution This enabledat a set ustemperature. to keep the This nutrient enabled into us the to keep initial the solution nutrient during into the the initial run solution and to stop during dissolution the run and cycle to atstop a set temperature by reversing the autoclave. Two values of GaPO concentration were found in the dissolution cycle at a set temperature by reversing the autoclave. Two4 values of GaPO4 concentration mixing 9 M H PO /6 M HCl (vol. proportion 95/5%) at temperatures 160–240 C: with and without were found in3 the4 mixing 9 M H3PO4/6 M HCl (vol. proportion 95/5%) at temperatures◦ 160–240 °C: crystallizationwith and without at the crystallization same T–P conditions at the (Figure same 2T–P) as wellconditions the kinetics (Figure of saturation 2) as well (Figure the 3kinetics). Likely, of this indicates a quite large zone of metastability of GaPO crystallization. The measured kinetics of saturation (Figure 3). Likely, this indicates a quite large zone4 of metastability of GaPO4 crystallization. saturationThe measured is relatively kinetics slow: of saturation more than is 100 relatively h before slow: it becomes more than constant. 100 h This before could it becomes be explained constant. by a lowThis convection could be explained or stationary by a conditions low convection in the or solution. stationary conditions in the solution.

Figure 2. GaPO4 concentration in 9 M H3PO4/ 6 M HCl (vol. proportion 95/5%) at temperature Figure 2. GaPO4 concentration in 9 M H3PO4/ 6 M HCl (vol. proportion 95/5%) at temperature 160– 160–240 C and pressure 5–20 bar. 1—with crystallization; 2—without crystallization. 240 °C and◦ pressure 5–20 bar. 1—with crystallization; 2—without crystallization.

The concentration of GaPO4 in 9 M H3PO4/12 M HCl solution (vol. proportion– 95/5%) showed a large difference between the values without and with crystallization (see Figure 1). This suggests that the starting conditions of GaPO4 crystal growth are very “soft”. Formation of different kinds of

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defects on the surface of seeds in a very beginning of crystallization are low or not desirable under such conditions. The influence of the low‐vapor‐pressure solvent additive, presented by HCl, changed the character of GaPO4 solubility compared to pure aqueous H3PO4, H2SO4 or their mixtures. The Molecules 2020, 25, 4518 4 of 14 concentration of GaPO4 in solutions used for the refluxing method is a constant at temperatures of 150–250 °C and pressures up to 10–20 bar.

Molecules 2020, 25, x 5 of 14 Figure 3. Kinetics of GaPO dissolution in 9 M H PO /6 M HCl (vol. proportion 95/5%) at 240 C and Figure 3. Kinetics of GaPO4 4 dissolution in 9 M H33PO4/6 M HCl (vol. proportion 95/5%) at 240 °C and ◦ 10–15 bar.10–155 M 1—with H bar.3PO 1—with4/12 crystallization; M crystallization; 2—without 2—without crystallization. crystallization. 6 HCl, 220 10 0.03 >0.04 – 30.0–18.5–1.34 The2.2. concentration Main Results97.5/2.5 of of the GaPO Crystal4 Growthin 9 M H3PO4/12 M HCl solution (vol. proportion– 95/5%) showed a large difference between the values without and with crystallization4 (see Figure1). This suggests More5 M H than3PO4 /1220 Mruns with durations up to 35 days of GaPO crystal growth were completed. 7 220 20 1.05 >0.86 * 10 16.6–11.2–8.3 that theCrystals starting HCl,were conditions 95/5 grown with of GaPO weights4 crystal up to 20 growth g. After are each very run, “soft”. the growth Formation rates of of crystals different and kinds of defects onconcentration the surface of ofGaPO seeds4 were in a measured. very beginning The crystal of crystallization habit and OH− arecontent low of or crystals not desirable (by IR under spectroscopy)5 M H3PO were4/12 Mdetermined. The main results of the runs are presented in Table 1. such conditions.8 230 15 0.09 0.13 0.4 14.5–10.2–4.4 The influenceHCl, of 95/5 the low-vapor-pressure solvent additive, presented by HCl, changed the character Table 1. Main results of GaPO4 crystal growth runs by the hydrothermal refluxing method. of GaPO4 solubility6 M H2SO compared4/12 M to pure aqueous H3PO4,H2SO4 or their mixtures. The concentration of 9 220 20 0.001 0.001 – – GaPO4 in solutionsHCl, 95/5 used for the refluxing method is a constantCrystal at temperatures of 150–250 ◦C and Solution Crystallization Growth Rates, Size of Grown pressures up to 10–20 bar. Gradient dT, 10N Composition,9 M H3PO4 Temperature,220 20 0 mm/day0 IR,– α Crystals,– °C 2.2. Main ResultsVol. ofProportion the Crystal Growth°C Y–X–Z mm 11 6 M H2SO4 225 20 Vx0 Vz0 – –

More than*—solution—the 20 runs with top durations part of the up autoclave—and to 35 days of GaPOthe nutrient4 crystal basket, growth in other were words, completed. the Crystals 9 M H3PO4/12 M were grown1 temperature with weights gradient. up to 20180 g. After each run,20 the growth0.44 rates>0.61 * of crystals5 26.8–16.5–4.2 and concentration HCl, 90/10 of GaPO4 were measured. The crystal habit and OH− content of crystals (by IR spectroscopy) were The face of basal pinacoid c{0001} is flat at temperature gradients up to 15 °C, it is represented determined. The9 M H main3PO4/12 results M of the runs are presented in Table1. >>0.37 by2 layer (Figure 4a). At gradients220 above 15–20 °C, the20 basal face0.28 is a poly‐head constructed0.8 47.5–11.3–5.7 by trigonal HCl, 95/5 * Thepyramids face of π basal’{0112 pinacoid} or z{0111 c{0001}} (Figure is 5a). ﬂat Growth at temperature rates of basal gradients pinacoids up were to 15 different◦C, it isfor represented the by layermentioned (Figure9 M 4Hcases:a).3PO4 Atat/12 low M gradients gradients, above it was up 15–20 to 0.2 mm/day◦C, the for basal two opposite face is faces; a poly-head at high gradients— constructed by 3 226 20 0.23 >0.44 * 0.5 33.8–18.0–7.9 trigonalmore pyramids than HCl,0.4 πmm/day 95/5’{01 12 }for or two z{01 opposite11} (Figure faces. 5a). Growth rates of basal pinacoids were di ﬀerent for the mentionedThe internal cases: structure at low of basal gradients, pinacoid it growth was up sectors to 0.2 was mm more/day homogenous for two opposite in the first faces; case at high under conditions9 M H3PO4 /12of low M temperature gradients (Figure 4b). At growth>>0.12 rates of 0.4 mm/day and more, gradients—more4 than 0.4 mm/day for236 two opposite16 faces. 0.21 0.4 49.7–19.7–11.3 the gas–liquidHCl, inclusions 95/5 and multiple dislocations were formed (Figure* 5b).

9 M H3PO4/12 M >>0.18 5 240 25 0.18 0.4 47.9–19.3–7.6 HCl, 95/5 *

(a) (b)

FigureFigure 4. (a) 4. Face (a) Face of basal of basal pinacoid pinacoid c{0001} c{0001} is is represented represented by layer by layer-dislocation‐dislocation growth mechanism, growth mechanism, at gradients up to 15 °C; (b) GaPO4 crystal grown on ZY seed plate and plates cut from this crystal: 1— at gradients up to 15 ◦C; (b) GaPO4 crystal grown on ZY seed plate and plates cut from this crystal: cut perpendicular to Y axis, 2—perpendicular to Z axis. 1—cut perpendicular to Y axis, 2—perpendicular to Z axis.

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Table 1. Main results of GaPO4 crystal growth runs by the hydrothermal reﬂuxing method.

Crystallization Crystal Growth Rates, mm/day Size of Grown Crystals, N Solution Composition, Vol. Proportion Gradient dT, C IR, α Temperature, C ◦ ◦ Vx Vz Y–X–Z mm

1 9 M H3PO4/12 M HCl, 90/10 180 20 0.44 >0.61 * 5 26.8–16.5–4.2

2 9 M H3PO4/12 M HCl, 95/5 220 20 0.28 >>0.37 * 0.8 47.5–11.3–5.7

3 9 M H3PO4/12 M HCl, 95/5 226 20 0.23 >0.44 * 0.5 33.8–18.0–7.9

4 9 M H3PO4/12 M HCl, 95/5 236 16 0.21 >>0.12 * 0.4 49.7–19.7–11.3

5 9 M H3PO4/12 M HCl, 95/5 240 25 0.18 >>0.18 * 0.4 47.9–19.3–7.6

6 5 M H3PO4/12 M HCl, 97.5/2.5 220 10 0.03 >0.04 – 30.0–18.5–1.34

7 5 M H3PO4/12 M HCl, 95/5 220 20 1.05 >0.86 * 10 16.6–11.2–8.3

8 5 M H3PO4/12 M HCl, 95/5 230 15 0.09 0.13 0.4 14.5–10.2–4.4

9 6 M H2SO4/12 M HCl, 95/5 220 20 0.001 0.001 – –

10 9 M H3PO4 220 20 0 0 – –

11 6 M H2SO4 225 20 0 0 – – *—solution—the top part of the autoclave—and the nutrient basket, in other words, the temperature gradient. MoleculesMolecules2020 2020, 25, 25,, 4518 x 6 of6 14 of 14 Molecules 2020, 25, x 6 of 14

(a) (b) Figure 5. (a) Poly‐head( aface) of basal pinacoid c{0001}, formed by trigonal rhombohedral(b) pyramids π’{0112} or z{0111}, gradients above 15–20 °C; (b) GaPO4 crystal grown on ZY seed plate. FigureFigure 5. 5.(a ()a Poly-head) Poly‐head face face of of basal basal pinacoid pinacoid c{0001}, c{0001}, formedformed byby trigonaltrigonal rhombohedral pyramids pyramids π’ 0112 0111 4 πCrystal’{01{ 12} or } growthor z{01 z{11}, on gradients}, gradients trigonal above aboveprism 15–20 15–20 faces◦C; °C; (wasb ()b GaPO) GaPOcharacterized4 crystal crystal grown grown by onhigh on ZYZY velocities, seedseed plate.plate. more than 1 mm/day. However, the crystals grown on x{1210} seeds were small because of trigonal pyramid faces TheCrystal internal growth structure on trigonal of basal prism pinacoid faces growth was characterized sectors was more by high homogenous velocities, in more the ﬁrst than case 1 appearance interrupting formation of negative and positive trigonal prism faces (Figure 6a,b). mm/day. However, the crystals grown on x{1210} seeds were small because of trigonal pyramid faces underCrystal conditions growth of low of all temperature trigonal rhombohedron gradients (Figure and4 trigonalb). At growth dipyramid rates faces of 0.4 was mm presented/day and more,by appearance interrupting formation of negative and positive trigonal prism faces (Figure 6a,b). thelayer gas–liquid or dislocation inclusions mechanism. and multiple Stairs dislocations(Figure 7a) and were low formed hillocks (Figure (Figure5b). 7b) were present on the rhombohedronCrystalCrystal growth growth faces. on of trigonalPropagation all trigonal prism rhombohedronof faces Brazil was twins characterized and from trigonal seed by highdipyramidwas velocities,observed faces morein was sectors than presented 1of mm the/ day.by However,rhombohedronlayer or thedislocation crystals faces grownmechanism.(Figure on 8a,b,c,d). x{12 Stairs10} Any seeds (Figure gas–liquid were 7a) small and inclusions, low because hillocks ofdislocations trigonal (Figure pyramid 7b)and were cracks faces present (see appearance Figure on the rhombohedron faces. Propagation of Brazil twins from seed was observed in sectors of the interrupting8b,c, d) were formation not typical of for negative sectors andof the positive rhombohedron trigonal prismface growth faces (Figureof GaPO6a,b).4 crystals. rhombohedron faces (Figure 8a,b,c,d). Any gas–liquid inclusions, dislocations and cracks (see Figure 8b,c, d) were not typical for sectors of the rhombohedron face growth of GaPO4 crystals.

(a) (b)

4 1210 FigureFigure 6. 6.GaPO GaPO4 crystals(a) grown on x{ x{1210} seeds, (a) (a) plate plate cut cut perpendicular perpendicular to( tob Y) Y axis, axis, b) ( btypical) typical habit habit ofof the the crystals crystals represented represented by by appearance appearance of of the the trigonal trigonal rhombohedralrhombohedral facesfaces interruptinginterrupting formation of negativeofFigure negative 6 and. GaPO and positive positive4 crystals trigonal trigonal grown prism onprism x{ faces.121 faces.0} seeds, (a) plate cut perpendicular to Y axis, b) typical habit of the crystals represented by appearance of the trigonal rhombohedral faces interrupting formation Crystalof negative growth and positive of all trigonal prism rhombohedron faces. and trigonal dipyramid faces was presented by layer or dislocation mechanism. Stairs (Figure7a) and low hillocks (Figure7b) were present on the rhombohedron faces. Propagation of Brazil twins from seed was observed in sectors of the rhombohedron faces (Figure8a–d). Any gas–liquid inclusions, dislocations and cracks (see Figure8b–d) were not typical for sectors of the rhombohedron face growth of GaPO4 crystals.

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(a) (b) (a) (b) Figure 7. (a) Stairs and (b) low hillocks of the rhombohedral faces < z > and < π’ >. FigureFigure 7. 7.( a(a)) Stairs Stairs andand ((bb)) lowlow hillocks of of the the rhombohedral rhombohedral faces faces < z and> and < π<’ >.π’ >.

Figure 8. X-ray topography of cross sections of GaPO4 crystals grown on c {0001} seed plates (thickness isFigure near to 8. 1.5 X mm)‐ray topography included some of cross defects: sections (a) twin, of GaPO (b) dislocation,4 crystals grown (c) crack on c and {0001} (d) all seed together. plates 1, Figure 8. X‐ray topography of cross sections of GaPO4 crystals grown on c {0001} seed plates 2—initial(thickness seed is plates;near to 3—defect1.5 mm) included (twin, dislocation some defects: or crack), (a) twin, 4—overgrown (b) dislocation, layer. (c) crack and (d) all (thicknesstogether. 1,is 2—initialnear to 1.5 seed mm) plates; included 3—defect some (twin, defects: dislocation (a) twin, or ( bcrack),) dislocation, 4—overgrown (c) crack layer. and (d) all 2.3. Growthtogether. Kinetics 1, 2—initial seed plates; 3—defect (twin, dislocation or crack), 4—overgrown layer. 2.4. Growth Kinetics 2.4. GrowingGrowth Kinetics of GaPO 4 crystals by the refluxing method in aqueous solution of H3PO4–HCl at Growing of GaPO4 crystals by the refluxing method in aqueous solution of H3PO4–HCl at temperatures of 200–240 ◦C and pressures up to 30 bar showed a strong influence of temperature Growing of GaPO4 crystals by the refluxing method in aqueous solution of H3PO4–HCl at gradienttemperatures to the of crystal 200–240 habit. °C and However, pressures the up growth to 30 ratesbar showed sequence a strong of diff influenceerent faces of oftemperature the crystals temperatures of 200–240 °C and pressures up to 30 bar showed a strong influence of temperature wasgradient constant. to the The crystal following habit. sequenceHowever, of the growth growth rates rates took sequence place of for different GaPO facescrystals of the obtained crystals by was the gradient to the crystal habit. However, the growth rates sequence of different faces4 of the crystals was refluxingconstant. method: The following sequence of growth rates took place for GaPO4 crystals obtained by the constant.refluxing The method: following sequence of growth rates took place for GaPO4 crystals obtained by the x{1210} > s{112 0} > r{1011} > c{0001} > π{1012} > z{0111} > π’{0112} refluxingx{121 method:0} > s{112 0} > r{1011} > c{0001} > π{101 2} > z{0111} > π’{0112} This growth rate sequence was in a good correlation with similar ones of GaPO4 crystals grown x{This121 0growth} > s{112 rate 0} sequence> r{1011} was> c{0001} in a good > π{101 correlation 2} > z{011 with1} > similar π’{011 ones2} of GaPO4 crystals grown from aqueous solutions of H2SO4 by the temperature gradient method at temperatures of 170–240 ◦C fromThis aqueous growth solutions rate sequence of H2SO was4 by in the a goodtemperature correlation gradient with similarmethod ones at temperatures of GaPO4 crystals of 170–240 grown °C and pressures of 10–30 bar. fromand pressuresaqueous solutions of 10–30 ofbar. H 2SO4 by the temperature gradient method at temperatures of 170–240 °C and pressures of 10–30 bar. 2.4. IR Spectroscopy 2.5. IR Spectroscopy 2.5. TheIR Spectroscopy influence of OH content on electromechanical properties of piezoelectric crystals is well The influence of OH−− content on electromechanical properties of piezoelectric crystals is well known [3,33]. It is more suitable to call it a hydrogen impurity which is frequently occurs forming knownThe [3,33]. influence It is ofmore OH suitable− content to on call electromechanical it a hydrogen impurity properties which of ispiezoelectric frequently occurs crystals forming is well a a bound to an oxygen and as a result forms the hydroxyl (OH ). This could be in different forms: knownbound [3,33].to an Itoxygen is more and suitable as a resultto call formsit a hydrogen the hydroxyl impurity (OH which−).− This is frequentlycould be in occurs different forming forms: a independent OH bonds—in unstructural impurities and OH− bonds in substitution to cations—as boundindependent to an oxygen OH− − bonds—in and as a unstructural result forms impurities the hydroxyl and (OHOH−− ).bonds This incould substitution be in different to cations—as forms: interstitialindependentinterstitial defects. defects. OH− Regardless,bonds—in Regardless, unstructural the the resulting resulting impurities OH OH− bond− bond and is highly isOH highly− bonds polar polar andin substitutionand as a as consequence a consequence to cations—as this dipolethis absorbsinterstitialdipole theabsorbs defects. infrared the Regardless,infrared radiation. radiation. the resulting OH− bond is highly polar and as a consequence this dipole absorbs the infrared radiation.

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1 It was found that the wave numbers near to 3400, 3290 and 3167 cm− of such absorption are the vibrations of OH− bonds of GaPO4 crystals [34–36]. For comparing the observed data to other 1 works [18,22,31,34–36] we used only the wave number at 3400 cm− . The first condition for precise determination of OH− content is a high structural homogeneity of crystals, absence of cracks and gas–liquid inclusions (mother solution impurity-free). As grown by the refluxing method, GaPO4 crystals satisfied this condition. The refluxing method applied to the growth of GaPO4 crystals showed that the solvents and T–P conditions similar to hydrothermal slow temperature gradient method allowed obtaining GaPO4 single crystals characterized by relatively low OH− content (Figure9) (at 220–240 ◦C and pressure up to 20 bar). Alpha (α) value (parameter for characterization of OH− content [34,35] of the grown GaPO4 single crystals by the refluxing method does not exceed 0.5. The α values of GaPO4 crystals grown by different methods are presented in Table2Molecules and Figure 2020, 259,. x 9 of 14

FigureFigure 9. IR spectra 9. IR spectra of GaPO of GaPO4 crystals4 crystals obtained obtained under under follow follow conditions: conditions: 1—hydrothermal 1—hydrothermal solution, solution, 9 9 M H3PO4M/12 HM3POHCl4/12 (80 M/ 20),HCl T (80/20),= 240 ◦ C,T = dT 240= 10°C,◦ C,dT refluxing = 10 °C, method, refluxing isometric method, crystals;isometric 2—hydrothermal crystals; 2— solution,hydrothermal 6 M H2SO 4solution,,T = 230 6 ◦MC, H dT2SO=4, 4T◦ =C, 230 temperature °C, dT = 4 °C, gradient temperature method, gradient plate-like method, crystals, plate‐ 3—flux,like crystals, 3—flux, lithium molybdate, T = 750–900 °C, decreasing of temperature, millimeter size lithium molybdate, T = 750–900 ◦C, decreasing of temperature, millimeter size crystals. crystals. Table 2. Comparative alpha values of GaPO4 crystals grown by temperature gradient, slow temperature rise,3. Materials refluxing and hydrothermal Methods methods and by flux method.

Crystal Growth 3.1. Main Features of theCrystal Hydrothermal Growth Refluxing Method Reproducibility Method Solution Rates, mm/day IR, α Temperature, ◦C (In Situ) The main features of the hydrothermal refluxing methodVx consists Vz of the following: a low filling of the autoclave with an initial solution (up to 50–60%); seeds and a nutrient material are placed in Slow temperature rise 150–185 9 M H3PO4 0.5–0.8 0.1–0.2 1.0–1.5 + two principally different zones of the autoclave; seeds are found in a solution (in the bottom of the 9 M Temperatureautoclave), gradientbut a nutrient is 176 above the solution.H PO /6 M Mass 0.5–0.8transport 0.05–0.1is carried out 0.5–1.0 by evaporation+/ of the 3 4 − solvent, condensation in the “cold” upperH2SO part4, 20 /of80 the autoclave (refrigerator) followed by diffusion Temperatureinto the nutrient gradient basket, dissolution 230 of 6 MtheH nutrientSO 0.8–1.5material 0.01–0.05and then, 0.04–0.1dropping down+/ into the 2 4 − solution in the bottom part of the autoclave.9 AsM a result of the multiple recirculation (refluxing) of the solvent,Reﬂuxing the initial solution 230 is graduallyH 3POsaturated4/12 M and0.1–1.2 maintains 0.1–1.0 this supersaturation<0.5 for+ the full crystal growth cycle. HCl, 95/5 Lithium TheFlux practical application 900–750 of the refluxing method0.25 for α‐GeO 0.052 crystal growth 0 and the+/ base of molybdate − previous studies on GaPO4 crystal growth have driven us to experimentally test the refluxing method to grow GaPO4 crystals.

3.2. Experimental Procedure For the first time, crystal growth vessels (autoclaves) were designed and approved for growing of GaPO4 by the refluxing method. A wide range of temperatures and solvents considered for the experiments led us to use platinum (Pt), tantalum (Ta) and polytetrafluoroethylene (PTFE) liners for the high pressure and temperature steel autoclaves. Precise temperature control was realized with the help of an internally inserted temperature control probe (thermocouple Type K). All autoclaves included a PTFE nutrient basket (Figure 10). Using the PTFE equipment prevented carrying out the experiments at temperatures above 250 °C. The estimated limiting factor for the hydrothermal refluxing method should be a critical temperature of a substance (the temperature at and above which vapor of the substance cannot be liquefied, no matter how much pressure). In our case, the limiting substances were aqueous solutions with well known critical temperature for water near 370 °C.

Molecules 2020, 25, 4518 9 of 14

3. Materials and Methods

3.1. Main Features of the Hydrothermal Refluxing Method The main features of the hydrothermal refluxing method consists of the following: a low filling of the autoclave with an initial solution (up to 50–60%); seeds and a nutrient material are placed in two principally different zones of the autoclave; seeds are found in a solution (in the bottom of the autoclave), but a nutrient is above the solution. Mass transport is carried out by evaporation of the solvent, condensation in the “cold” upper part of the autoclave (refrigerator) followed by diffusion into the nutrient basket, dissolution of the nutrient material and then, dropping down into the solution in the bottom part of the autoclave. As a result of the multiple recirculation (refluxing) of the solvent, the initial solution is gradually saturated and maintains this supersaturation for the full crystal growth cycle. The practical application of the refluxing method for α-GeO2 crystal growth and the base of previous studies on GaPO4 crystal growth have driven us to experimentally test the refluxing method to grow GaPO4 crystals.

3.2. Experimental Procedure For the first time, crystal growth vessels (autoclaves) were designed and approved for growing of GaPO4 by the refluxing method. A wide range of temperatures and solvents considered for the experiments led us to use platinum (Pt), tantalum (Ta) and polytetrafluoroethylene (PTFE) liners for the high pressure and temperature steel autoclaves. Precise temperature control was realized with the help of an internally inserted temperature control probe (thermocouple Type K). All autoclaves included a MoleculesPTFE nutrient 2020, 25, basketx (Figure 10). Using the PTFE equipment prevented carrying out the experiments10 of 14 at temperatures above 250 ◦C. The estimated limiting factor for the hydrothermal refluxing method shouldAccording be a critical to the temperature known conditions of a substance of GaPO (the4 crystallization, temperature in at order and above to obtain which the vapor lowest of OH the− contentsubstance in cannotthe crystals, be liquefied, the most no suitable matter solvents how much are pressure). aqueous H In3PO our4, case,H2SO the4 and limiting HCl solutions substances or theirwere mixtures aqueous solutionsand temperatures with well exceeding known critical 200 °C. temperature These temperatures for water nearcorrespond 370 ◦C. to retrograde or constant solubility of GaPO4 (see Figure 1).

Figure 10. (1,2) Ta-Ta‐ andand (3)(3) Pt-lined Pt‐lined steel steel autoclaves autoclaves with with PTFE PTFE cover; cover; (4) (4) autoclaves autoclaves equipped equipped with with an an(5) (5) internal internal temperature temperature control control probe probe and and (6) PTFE(6) PTFE nutrient nutrient basket. basket. Pen Pen for for scale. scale.

According to the known conditions of GaPO crystallization, in order to obtain the lowest OH This is why we cannot use a unsaturated solution4 for starting crystal growth of GaPO4 crystals,− ascontent it risks in dissolving the crystals, seeds. the most In other suitable words, solvents the initial are aqueous (starting) H solution3PO4,H 2mustSO4 andbe already HCl solutions saturated or their mixtures and temperatures exceeding 200 C. These temperatures correspond to retrograde or with GaPO4 at the precise concentration of GaPO◦ 4 corresponding to the equilibrium temperature to avoidconstant spontaneous solubility ofcrystallization GaPO4 (see Figure or dissolution1). of the seed. Improvements of the refluxing method used for α‐GeO2 crystal growth concerning conditions of GaPO4 crystal growth has allowed carrying out the saturation of initial solution and further growth of GaPO4 crystals in the same run cycle. This does not interrupt the course of the run, and as a consequence, neither spontaneous crystals nor losing of seed were observed. Technically, the following steps are in the first part of run: the autoclave is charged by solvent, nutrient and seeds are installed with the seeds on the top and nutrient below. The temperature is 10–30 °C higher in the top of autoclave (Figure 11a) than the bottom (to prevent evaporated solution convection). In the bottom, a working temperature of the future crystal growth is set. After completing the saturation of the solution by GaPO4, the autoclave is turned over. Thus, seeds take hold in the saturated solution (Figure 11b) at equilibrium temperature, and the nutrient remains above them. The difference in temperatures between the mother solution (bottom) and the refrigerator (top part of the autoclave) initiates the evaporation of the solvent and its condensation on the surface of the refrigerator. The process continues, as described before, as the condensate flows to the nutrient basket where the dissolution of GaPO4 nutrient occurs, followed by the dropping down of the saturated solution from the nutrient basket to the bottom part of the autoclave. The recirculation of the solvent in the given cycle results in a gradual super saturation of the initial solution. It maintains the saturation of the mother solution necessary for GaPO4 crystals growth at all times of the run. The principal schema of the refluxing system in an autoclave is shown in Figure 12. As reported by [10,18,29,31], good quality crystals of GaPO4 have been grown in aqueous solutions of (H3PO4 and H2SO4) acids at temperatures of 170–260 °C and pressures up to 50 bar. We used the same solutions as mineralizers in our experiments. However, the vapor pressure of such acids that excludes a formation of condensate in sufficient quantities at temperatures up to 250 °C is low (Figure 13a,b). Consequently, solutions of H3PO4 and H2SO4 could not be a solvent for the nutrient. The addition of low‐vapor‐pressure solvents, for example, hydrochloric acid (HCl) to the initial solution allowed increasing nutrient dissolution under vapor/condensate recirculation conditions.

Molecules 2020, 25, 4518 10 of 14

This is why we cannot use a unsaturated solution for starting crystal growth of GaPO4 crystals, as it risks dissolving seeds. In other words, the initial (starting) solution must be already saturated with GaPO4 at the precise concentration of GaPO4 corresponding to the equilibrium temperature to avoid spontaneous crystallization or dissolution of the seed. Improvements of the refluxing method used for α-GeO2 crystal growth concerning conditions of GaPO4 crystal growth has allowed carrying out the saturation of initial solution and further growth of GaPO4 crystals in the same run cycle. This does not interrupt the course of the run, and as a consequence, neither spontaneous crystals nor losing of seed were observed. Technically, the following steps are in the first part of run: the autoclave is charged by solvent, nutrient and seeds are installed with the seeds on the top and nutrient below. The temperature is 10–30 ◦C higher in the top of autoclave (Figure 11a) than the bottom (to prevent evaporated solution convection). In the bottom, a working temperature of the future crystal growth is set. After completing the saturation of the solution by GaPO4, the autoclave is turned over. Thus, seeds take hold in the saturated solution (Figure 11b) at equilibrium temperature, and the nutrient remains above them. The difference in temperatures between the mother solution (bottom) and the refrigerator (top part of the autoclave) initiates the evaporation of the solvent and its condensation on the surface of the refrigerator. The process continues, as described before, as the condensate flows to the nutrient basket where the dissolution of GaPO4 Moleculesnutrient 2020 occurs,, 25, x followed by the dropping down of the saturated solution from the nutrient basket11 of 14 to the bottom part of the autoclave. The recirculation of the solvent in the given cycle results in a gradual In summary, the real process of mass transport could be represented as: a mother solution in the super saturation of the initial solution. It maintains the saturation of the mother solution necessary bottom part of the autoclave, represented by “heavy” (relatively low vapor‐pressure) acids as the for GaPO4 crystals growth at all times of the run. The principal schema of the refluxing system in an mineralizer and a low‐vapor‐pressure acid as the conveyor of dissolved GaPO4 from the nutrient to autoclave is shown in Figure 12. the mother solution (see Figure 12).

Figure 11. Runs sketch, saturation of the initial solution and further growth of GaPO4 crystals in the same run cycle. (a) First part of the run, autoclave is installed with the seeds on the top and nutrient– in the down; (b) second part of the run: autoclave is turned over, the seeds take a place in the saturated Figuresolution 11. and Runs the sketch, nutrient saturation remains above.of the initial solution and further growth of GaPO4 crystals in the same run cycle. (a) First part of the run, autoclave is installed with the seeds on the top and nutrient– in the down; (b) second part of the run: autoclave is turned over, the seeds take a place in the saturated solution and the nutrient remains above.

Molecules 2020, 25, x 11 of 14

In summary, the real process of mass transport could be represented as: a mother solution in the bottom part of the autoclave, represented by “heavy” (relatively low vapor‐pressure) acids as the mineralizer and a low‐vapor‐pressure acid as the conveyor of dissolved GaPO4 from the nutrient to the mother solution (see Figure 12).

Figure 12. Principal sketch of the reﬂuxing system in an autoclave. Mass transport in the reﬂuxing system: a mother solution is in the bottom part of the autoclave, presented by “heavy” acids as the

mineralizer, and a low-vapor-pressure acid is the conveyor of dissolved GaPO4 from nutrient to the mother solution.

As reported by [10,18,29,31], good quality crystals of GaPO have been grown in aqueous solutions Molecules 2020, 25, x 4 12 of 14 of (H3PO4 and H2SO4) acids at temperatures of 170–260 ◦C and pressures up to 50 bar. We used the sameFigure solutions 12. Principal as mineralizers sketch of the inrefluxing our experiments. system in an However,autoclave. Mass the vapor transport pressure in the ofrefluxing such acids that excludessystem: a amother formation solution of condensateis in the bottom in supartﬃ cientof the quantities autoclave, atpresented temperatures by “heavy” up to acids 250 as◦C the is low

(Figuremineralizer, 13a,b). Consequently, and a low‐vapor solutions‐pressure of acid H 3isPO the4 andconveyor H2SO of4 dissolvedcould not GaPO be a4 solvent from nutrient for the to nutrient. the The additionmother solution. of low-vapor-pressure solvents, for example, hydrochloric acid (HCl) to the initial solution allowed increasing nutrient dissolution under vapor/condensate recirculation conditions.

(a) (b)

FigureFigure 13.13.( a()a Partial) Partial pressure pressure of H 2ofSO H4 over2SO4 aqueousover aqueous sulfuric sulfuric acid [37]; acid (b) pressure [37]; (b versus) pressure temperature versus attemperature ﬁll of 82% forat fill H2 ofO, 82% 7.58 for M HH32POO, 7.584 and M 7.58 H3PO M4 H and3PO 7.584 saturated M H3PO with4 saturated AlPO4 with[38]. AlPO4 [38].

Plates (with sizes up to 45 × 15 × 1 mm) cut from GaPO4 crystals grown by slow temperature rise method in aqueous solution of 9 M H3PO4 at temperatures of 150–165 °C and pressure up to 5 bar were used as seeds. Different crystallographic orientations c{0001}, s{112 0}, r{1011}, z{0111}, π{101 2}, π’{0112} and x{1210} were cut. The morphology and internal structure of the obtained GaPO4 crystals were studied on the surface and in polished cuts by optical microscopyMBS‐9 (Russia) , ADF STD16 (China), Nikon Eclipse LV100Pol (Japan). The IR spectroscopy of the crystals or polished plates was performed on PerkinElmer 820 spectrometer (USA). The OH− content in crystals were estimate by α value, which was calculated using the formula α = (1/l)*(Log(T3800/T3400))‐0,078, where: l—thickness of sample in cm, T3800 and T3400—intensity of absorption at wave numbers of 3800 and 3400 cm−1, 0.078— absorption of the intrinsic lattice vibrations of GaPO4 at 3400 cm−1 [34,35]. The presence of defects in the crystal plates was identify by X‐ray topography at IMPMC UMR CNRS 7590 (Montpellier, France) by a homemade device using a Cu Kα tube.

4. Conclusions

The GaPO4 crystals grown by the hydrothermal refluxing method at temperatures close to 240 °C and pressures up to 10–20 bar in aqueous solutions of the H3PO4–HCl mixtures have a relatively low OH− content and considerably high structural uniformity in comparison to crystals grown by other low temperature (< 250 °C) hydrothermal methods. Hydrothermal refluxing method of GaPO4 crystals growth has shown a very high reproducibility of the results. Improvement of the hydrothermal refluxing method concerning application to GaPO4 crystal growth gave a new technique of measurement of GaPO4 concentration in solution at set parameters. The presented technique allows increasing the accuracy of GaPO4 concentration definition in aqueous solutions of H3PO4, H2SO4, HCl and their mixtures at least up to 250 °C and pressures of 10–30 bar.

Molecules 2020, 25, 4518 12 of 14

In summary, the real process of mass transport could be represented as: a mother solution in the bottom part of the autoclave, represented by “heavy” (relatively low vapor-pressure) acids as the mineralizer and a low-vapor-pressure acid as the conveyor of dissolved GaPO4 from the nutrient to the mother solution (see Figure 12). Plates (with sizes up to 45 15 1 mm) cut from GaPO crystals grown by slow temperature rise × × 4 method in aqueous solution of 9 M H3PO4 at temperatures of 150–165 ◦C and pressure up to 5 bar were used as seeds. Diﬀerent crystallographic orientations c{0001}, s{112 0}, r{1011}, z{0111}, π{101 2}, π’{0112} and x{1210} were cut. The morphology and internal structure of the obtained GaPO4 crystals were studied on the surface and in polished cuts by optical microscopyMBS-9 (Russia), ADF STD16 (China), Nikon Eclipse LV100Pol (Japan). The IR spectroscopy of the crystals or polished plates was performed on PerkinElmer 820 spectrometer (USA). The OH− content in crystals were estimate by α value, which was calculated using the formula α = (1/l)*(Log(T3800/T3400))-0,078, where: l—thickness of sample in cm, T3800 and 1 T3400—intensity of absorption at wave numbers of 3800 and 3400 cm− , 0.078—absorption of the 1 intrinsic lattice vibrations of GaPO4 at 3400 cm− [34,35]. The presence of defects in the crystal plates was identify by X-ray topography at IMPMC UMR CNRS 7590 (Montpellier, France) by a homemade device using a Cu Kα tube.

4. Conclusions

The GaPO4 crystals grown by the hydrothermal refluxing method at temperatures close to 240 ◦C and pressures up to 10–20 bar in aqueous solutions of the H3PO4–HCl mixtures have a relatively low OH− content and considerably high structural uniformity in comparison to crystals grown by other low temperature (< 250 ◦C) hydrothermal methods. Hydrothermal refluxing method of GaPO4 crystals growth has shown a very high reproducibility of the results. Improvement of the hydrothermal refluxing method concerning application to GaPO4 crystal growth gave a new technique of measurement of GaPO4 concentration in solution at set parameters. The presented technique allows increasing the accuracy of GaPO4 concentration definition in aqueous solutions of H3PO4,H2SO4, HCl and their mixtures at least up to 250 ◦C and pressures of 10–30 bar.

Author Contributions: Conceptualization, D.B. and E.P.; methodology, V.B., L.B; validation E.P., V.B., P.P.; formal analysis, D.B., L.B., T.S., T.B.; investigation D.B., T.S., L.B., T.B.; writing—original draft preparation, D.B.; writing—review and editing, D.B., T.S., T.B.; supervision, P.P.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by RFBR Grant Number 17-05-00976 Acknowledgments: We are grateful to colleagues from the former ICGM, CNRS-UM-ENSCM, UMR 5253, Université Montpellier for collaboration and, personally, to Jacques Detaint (IMPMC UMR CNRS 7590) for friendly help on X-ray characterization of the samples. A part of this work is fulfilled under the Research Program AAAA-A18-118020590150-6 of the D.S. Korzhinskii IEM RAS. Conflicts of Interest: The authors declare no conflicts of interest. In Memory of Professor Nikolay Leonyuk: For all of us he will be missed. Inspired by his modesty, determination and honesty we will remember him as a great crystal grower, educator and a true gentleman.

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Sample Availability: Samples of the compounds GaPO4 are available from the authors.

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