crystals Review Hydrothermal Crystal Growth of Piezoelectric α-Quartz Phase of AO2 (A = Ge, Si) and MXO4 (M = Al, Ga, Fe and X = P, As): A Historical Overview Olivier Cambon * and Julien Haines Institut Charles Gerhardt Montpellier, Centre National de la Recherche Scientifique, Université de Montpellier, Ecole Nationale Supérieure de Chimie de Montpellier, Montpellier 34095 Cedex 05, France; [email protected] * Correspondence: [email protected]; Tel.: +33-4-67-14-32-04 Academic Editors: Alain Largeteau and Mythili Prakasam Received: 19 November 2016; Accepted: 25 January 2017; Published: 4 February 2017 Abstract: Quartz is the most frequently used piezoelectric material. Single crystals are industrially grown by the hydrothermal route under super-critical conditions (150 MPa-623 K). This paper is an overview of the hydrothermal crystal growth of the AO2 and MXO4 α-quartz isotypes. All of the studies on the crystal growth of this family of materials enable some general and schematic conclusions to be made concerning the influence of different parameters for growing these α-quartz-type materials with different chemical compositions. The solubility of the material is the main parameter, which governs both thermodynamic parameters, P and T, of the crystal growth. Then, depending on the chemistry of the α-quartz-type phase, different parameters have to be considered with the aim of obtaining the basic building units (BBU) of the crystals in solution responsible for the growth of the α-quartz-type phase. A schematic method is proposed, based on the main parameter governing the crystal growth of the α-quartz phase. All of the crystal growth processes have been classified according to four routes: classical, solute-induced, seed-induced and solvent-induced crystal growth. Keywords: hydrothermal crystal growth; α-quartz isotypes; basic building units (BBU) of the crystal; nucleation; homo-epitaxy; hetero-epitaxy; solute; seed; solvent 1. Introduction Hydrothermal growth is a natural process and occurs in the Earth’s crust. Originally, this term was used to describe the dissolution of rocks and minerals by water, under elevated pressures and temperatures, and the crystallization of various inorganic phases. Beryl, calcite, and quartz are the well-known large crystals, which are extracted from the Earth. Due to the interest in such materials, the hydrothermal technique was used to produce industrially synthetic materials, like quartz, for electronic applications [1,2]. Synthetic beryl and calcite single crystals have also been grown hydrothermally [3–6]. The term hydrothermal is now used to describe crystallization processes using aqueous solvents in a closed vessel. Based on this approach, sub- or super-critical conditions have been used in the vessel, depending on the solvent. This paper presents a review of the hydrothermal synthesis of the α-quartz IV III V phase in A O2 (A = Si, Ge) and M X O4 (M = Al, Ga and X = P, As) compounds. The α-quartz type structure belongs to the P3121 or P3221 space group. AO4 (A = Si and/or Ge for SiO2, GeO2 or Si1−xGexO2), MO4, and XO4 (for MXO4) tetrahedral units are connected, forming a helical chain along the c axis (Figure1). The α-quartz phase is stable at room temperature for all of the compounds except for GeO2, for which the α-quartz phase is stable at high temperatures from 1306 K up to the melting point at 1388 K (Table1): the α-quartz-type phase of GeO2 is a thermodynamic metastable phase at Crystals 2017, 7, 38; doi:10.3390/cryst7020038 www.mdpi.com/journal/crystals Crystals 2017, 7, 38 2 of 13 Crystals 2016, 6, 38 2 of 12 room temperature. The stable phase of GeO2 under ambient conditions is a rutile-type phase, in which phasethe germanium at room temperature. is 6-fold coordinated. The stable phase of GeO2 under ambient conditions is a rutile-type phase, in which the germanium is 6-fold coordinated. MXO4 AO2 2c c A O Figure 1. α-quartz structure of AO2 (A = Si, Ge) and MXO4 (M = Al, Ga, Fe and X = P, As) materials. Figure 1.α-quartz structure of AO2 (A = Si, Ge) and MXO4 (M = Al, Ga, Fe and X = P, As) materials. Table 1. Polymorphism of the AO2 and MXO4 materials. Table 1. Polymorphism of the AO2 and MXO4 materials. CompoundCompound Phase Phase Transitions Transitions 846 K 870 K 1673 K 1983 K SiO ⎯846⎯⎯ K→ ⎯870⎯⎯ K→ ⎯1673⎯⎯ K→ 2 α-Quartzα-Quartz ←⎯ −−−−−!−−−−− ⎯⎯β -Quartzβ-Quartz −−−−−!−−−−− ←⎯Tridymite ⎯⎯ Tridymite −−−−−!−−−−− β -Cristobalite←⎯ ⎯⎯ −−−−−! −−−−−β-Cristobaliteliquid SiO2 1306 K 1388 K GeO2 β-Rutile −−−−−!−−−−−⎯⎯⎯1983 Kα→-Quartz −−−−−!−−−−− Liquid 859 K 1088←⎯ K ⎯⎯ liquid1298 K 1870 K AlPO4 α-Quartz −−−−−! β-Quartz −−−−−! Tridymite −−−−−! β-Cristobalite −−−−−! liquid −−−−− ⎯⎯⎯1306 −−−−− K→ −−−−−⎯⎯⎯1388 K→ −−−−− GeO2 ←⎯ ⎯⎯980 K ←⎯1513 K ⎯⎯ FePO4 β-Rutileα -Quartz −−−−−!−−−−− α-Quartzβ-Quartz −−−−−!−−−−− Liquid Liquid ⎯859⎯⎯ K→ 1206⎯ K 1088⎯⎯ K→ 1943 K ⎯1298⎯⎯ K→ GaPO4 α-Quartz ←⎯ ⎯⎯ α -Quartzβ-Quartz −−−−−!−−−−− ←⎯β ⎯-Cristobalite⎯ Tridymite −−−−−!−−−−− ←⎯Liquid ⎯⎯ β-Cristobalite AlPO4 1303 K GaAsO4 α-Quartz −−−−−!⎯⎯⎯−−−−−1807 K→Chemical decomposition ←⎯ ⎯⎯ liquid ⎯⎯⎯980 K→ ⎯⎯⎯1513 K→ 2. HydrothermalFePO4 Crystal Growthα-Quartz ←⎯ ⎯⎯ β-Quartz ←⎯ ⎯⎯ Liquid ⎯1206⎯⎯ K→ ⎯1943⎯⎯ K→ GaPOFollowing4 the initial workα of-Quartz Gibbs [7←⎯] in 1878, ⎯⎯ and β-Cristobalite then that of Frenkel←⎯ ⎯ [⎯8] in Liquid 1945, Burton, Cabrera, and Frank [9,10] studied the growth of a real crystal⎯⎯⎯1303 K from→ the vapor phase. Thermodynamic equations GaAsO4 α-Quartz ←⎯ ⎯⎯ Chemical decomposition have been established, which describe the crystal growth from a structural defect, like a screw-type dislocation. Numerous authors [11–13] have applied this theory to growth from solution. Cabrera and 2. Hydrothermal Crystal Growth Levine [14] extended this approach to dissolution, which is the reverse phenomenon with respect to crystalFollowing growth. the All ofinitial these work studies of Gibbs were reconsidered[7] in 1878, and by Johnstonthen that inof 1962 Frenkel [15]. [8] Basically, in 1945, in Burton, crystal Cabrera,growth from and solution,Frank [9,10] the solutestudied crystallizes the growth when of a real the concentrationcrystal from the of thevapor solute phase. becomes Thermodynamic higher than equationsthe concentration have been at equilibrium, established, termedwhich describe solubility. th Thee crystal solubility growth is the from main a structural parameter defect, for growing like a screw-typecrystals from dislocation. solutions. HydrothermalNumerous authors crystal [11–13] growth ha correspondsve applied this to crystaltheory growthto growth from from solution solution. in a Cabreraclosed vessel. and Levine Temperature [14] extended and pressure this approach are two important to dissolution, thermodynamic which is the parameters, reverse phenomenon which have withto be respect adjusted to according crystal growth. to the solventAll of these used. st Theudies first were step reconsidered is to find the adequateby Johnston thermodynamic in 1962 [15]. Basically,conditions in for crystal obtaining growth a good from reactivity solution, ofthe the solute solvent crystallizes with the when solute. the Then, concentration for growing of crystalsthe solute of becomeshigh quality, higher the than experimental the concentration conditions at have equilibriu to be adjustedm, termed to solubility. modify the The kinetics solubility of the is exchanges the main parameterbetween the for solid growing phase crystals and the from solvent. solutions. By using Hydrothermal hydrothermal crystal techniques, growth thermodynamic-stablecorresponds to crystal growthand metastable from solution phases canin bea grown.closed Thevessel. main Temperature experimental and parameters, pressure which are cantwo beimportant adjusted thermodynamicare the solvent, theparameters, presence orwhich absence have of to a be nucleus adjusted (or according seed) and bothto the thermodynamic solvent used. The parameters, first step isP andto find T. the adequate thermodynamic conditions for obtaining a good reactivity of the solvent with the solute.One of Then, the mostfor growing important crystals questions, of high whichquality, has the to experimental be solved before conditions starting have a hydrothermalto be adjusted process,to modify is thethe choicekinetics of of which the pressureexchanges domain between the the crystal solid growth phase process and the will solvent. be performed By using in. Tohydrothermal answer to thistechniques, question, thermodynamic-stable the ability of the solvent and metastable to dissolve phases the chemical can be precursorsgrown. The and main to experimental parameters, which can be adjusted are the solvent, the presence or absence of a nucleus (or seed) and both thermodynamic parameters, P and T. Crystals 2017, 7, 38 3 of 13 recrystallize the desired phase has to be considered. In a polar solvent like aqueous solutions, the dissolution process forms ionic species in solution and crystallization is due to the condensation of the ionic species to form a new crystal. In pure water (pH = 7), H+ and OH− species are present with a concentration of 10−7 M. IV 2.1. Hydrothermal Growth of A O2 (A = Si, Ge) In the Earth, crystal growth of quartz occurs in water, which acts as the solvent under hydrothermal conditions. In the supercritical state, water is more aggressive due to the decrease in its ionic product [16] and the growth of quartz single crystals can take place, but with very slow kinetics. For the industrial production of quartz crystals, sodium hydroxide