Bull. Mater. Sci., Vol. 34, No. 6, October 2011, pp. 1257–1262. c Indian Academy of Sciences. Ba3(P1−xMnxO4)2 : Blue/green inorganic materials based on tetrahedral Mn(V) SOURAV LAHA, ROHIT SHARMA, S V BHAT†, MLPREDDY‡, J GOPALAKRISHNAN∗ and S NATARAJAN Solid State and Structural Chemistry Unit, †Department of Physics, Indian Institute of Science, Bangalore 560 012, India ‡Chemical Science and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST), Thiruvananthapuram 695 019, India MS received 11 May 2011 ) 3− 2 Abstract. We describe a blue/green inorganic material, Ba3(P1−xMnxO4 2 (I) based on tetrahedral MnO4 :3d chromophore. The solid solutions (I) which are sky-blue and turquoise-blue for x ≤ 0·25 and dark green for x ≥ 0·50, 3− are readily synthesized in air from commonly available starting materials, stabilizing the MnO4 chromophore in an isostructural phosphate host. We suggest that the covalency/ionicity of P–O/Mn–O bonds in the solid solutions tunes the crystal field strength around Mn(V) such that a blue colour results for materials with small values of x. The material could serve as a nontoxic blue/green inorganic pigment. Keywords. Blue/green inorganic material; barium phosphate/manganate(V); tetrahedral manganate(V); blue/green chromophore; ligand field tuning of colour. 1. Introduction field transitions within an unusual five coordinated trigo- nal bipyramidal Mn(III). A turquoise-blue solid based on Inorganic solids displaying bright colours are important Li1·33Ti1·66O4 spinel oxide wherein the colour arises from as pigment materials which find a wide range of appli- an intervalence charge-transfer between Ti3+ and Ti4+ has cations in paints, inks, plastics, rubbers, ceramics, ena- also been described recently (Fernández-Osorio et al 2011). mels and glasses (Buxbaum and Pfaff 2005). From a Considering the paucity of blue/green inorganic solids that scientific standpoint, there are atleast three different mecha- are colour-stable, environmentally benign and nontoxic, as nisms by which inorganic solids acquire colour: (i) elec- well as readily synthesized from cost-effective commonly tronic interband transitions in the visible (Jansen and available constituents, we perceived the need to develop Letschert 2000; Dolgos et al 2009) as for example in new blue or blue–green inorganic pigment-quality mate- the recently discovered yellow-red pigments based on pe- rials. Towards this end, we identified tetrahedral Mn(V): 3− rovskite, (Ca,La)Ta(O,N)3 (Jansen and Letschert 2000), MnO4 as a blue-green chromophore that could be rea- (ii) intervalence charge-transfer between transition metal dily stabilized in a barium phosphate lattice. While the + + ions as in the blue sapphire (Fe2 –Ti4 charge-transfer in Mn(V):MnO3− ion is known to be stable in several alka- α 4 -Al2O3 host) (http://en.wikipedia.org/wiki/Sapphire) and line earth phosphate lattices (Kingsley et al 1965; Borromei (iii) crystal field transitions as in the well known ruby et al 1981; Dardenne et al 1998), we found that Ba3(PO4)2 3+ α ) (octahedrally coordinated Cr in -Al2O3 (Esposti and is a convenient matrix for developing a blue/green pig- Bizzocchi 2007). 3− ) ment based on Mn(V):MnO4 , inasmuch as both Ba3(PO4 2 Among the three primary colours (red, green and blue) that (white) and Ba3(MnO4)2 (intense green) are isostructural produce different shades of colours in inorganic solids, blue (Zachariasen 1948; Sugiyama and Tokonami 1990; Weller coloured pigment quality materials are relatively rare. Some and Skinner 1999), crystallizing in a hexagonal R–3m struc- 3− of the common blue pigments of inorganic origin are : azurite ture that contains discrete PO4/MnO4 tetrahedra. MnO ) ) 4 [Cu3(CO3 2(OH)2], Egyptian blue (CaCuSi4O10 , Prussian acts as the chromophore in the isostructural solid solutions blue (Fe [Fe(CN )] ), and ultramarine (Na Al Si O S ). 4 6 3 7 6 6 24 3 between Ba3(PO4)2 and Ba3(MnO4)2, where we are able to YIn1−x Mnx O3 is a recently developed blue pigment tune the colour from sky-blue for low concentrations of Mn (Smith et al 2009) that derives its colour from crystal to turquoise blue and to dark green with increasing Mn con- centration across the series, Ba3(P1−x Mnx O4)2. The synthe- sis and characterization of structure and optical properties of this material identifying the lattice tuning of colour are ∗Author for correspondence ([email protected]) described in this paper. 1257 1258 Sourav Laha et al 2. Experimental BaCO3,(NH4)H2PO4 and MnC2O4·2H2O were mixed in stoichiometric proportions and the mixtures heated first at 2.1 Sample preparation 400◦C for 4 h, followed by heating at 950◦C with intermit- tent grindings. The duration of the 950◦C heating depends on ) = · · · · · · Ba3(P1−x Mnx O4 2 (x 0, 0 02, 0 05, 0 10, 0 25, 0 30, 0 40, the composition. Thus, for 2% and 5% Mn doping, a heat- 0·50, 0·75, 0·90 and 1·00) compounds were prepared by ingof12h× 2 was adequate. Higher concentration of Mn conventional ceramic method. For this purpose, high purity doping required longer heating times (up to 12 h × 7) at 950◦C. 2.2 Experimental techniques ) 3 600 Å Formation of single-phase products and their crystal struc- ture were studied by powder X-ray diffraction (PXRD). 590 PXRD patterns were recorded with a Philips X’pert Diffrac- tometer (Ni-filtered Cu Kα radiation, λ = 1·5418 Å). PXRD 580 patterns were simulated by the program POWDERCELL (Kraus and Nolze 1996). Cell parameters were determined Cell volume ( volume Cell by Le Bail fitting of the data by means of the program GSAS 570 (Larson and Von Dreele 1990). PXRD data for Rietveld refinement of the structures were collected at room tempe- 5.85 θ 21.4 rature employing the same diffractometer in the 2 range 20–60◦ withastepsizeof0·02◦ and step duration, 25 s. 5.80 The refinement was carried out by means of the program 21.3 c ( ) 5.75 GSAS (Larson and Von Dreele 1990). A sixth order Cheby- Å Å 21.2 ) chev polynomial for the background, zero, LP factor, scale, a ( 5.70 pseudo-Voigt profile function (U, V , W and X), lattice 5.65 21.1 parameters, atomic parameters, and Uiso (total 23 parame- ters) were used in refinement. The thermal parameters were 5.60 21.0 constrained to be the same for atoms occupying the same site 0.0 0.2 0.4 0.6 0.8 1.0 (Mn and P). Diffuse reflectance spectra of powdered samples were x in Ba3(P 1-xMnxO4)2 recorded by means of a Perkin Elmer Lambda 750 UV-vis spectrometer in 300–1300 nm range. For characterization Figure 1. Unit cell parameters of Ba3(P1−x Mnx O4)2 solid solu- of pigment quality of the samples (the colour parameters), tions: (bottom) a and c and (top) cell volume. the diffuse reflectance spectra were recorded (380–780 nm) Intensity (a. u.) Figure 2. Rietveld refinement of structure of Ba3(P0·25Mn0·75O4)2 from pow- der XRD data. Observed (+), calculated (−) and difference (bottom) profiles are shown. Vertical bars indicate position of Bragg reflections. Ba3(P1−x Mnx O4)2 : Blue/green inorganic materials based on tetrahedral Mn(V) 1259 Table 1. Crystallographic data for Ba3(P0·25Mn0·75O4)2 [space group R–3m, a = 5·6773(1) Å, c = 21·3156(7) Å] together with selected bond lengths. Atom Site xy zUiso Occupancy Ba1 3a 0 0 0 0·033(2) 1 Ba2 6c 0 0 0·2076(1) 0·017(1) 1 P6c0 00·4070(1) 0·010(2) 0·25 Mn 6c 0 0 0·4070(1) 0·010(2) 0·75 O1 6c 0 0 0·3292(1) 0·024(8) 1 O2 18h 0·163(1) 0·326(2) 0·5653(5) 0·012(4) 1 = · = · 2 = · χ2 = · Reliability factors: Rp 7 59, Rwp 10 95, RF 7 00, 1 753; bond lengths (Å): Ba(1) − O(1) = 3·279 (1)(×6); Ba(1) − O(2) = 2·730 (1)(×6); Ba(2) − O(1) = 2·591(3); Ba(2) − O(2) = 2·886(2)(×6); Ba(2) − O(2) = 2·763 (1)(×3); P/Mn − O(1) = 1·660(1); P/Mn − O(2) = 1·710(1)(×3) employing a Shimadzu UV-2450 UV-vis spectrometer UV-2450 spectrometer. The CIE 1976 L*a*b* colorime- equipped with an integrating sphere attachment, ISR- tric method was used, as recommended by the Commission 2200. The measurements were made with an illuminant Internationale de l’Eclairage (CIE). In this method, L*is ◦ D65,10 complementary observer and measuring geo- the lightness axis [black (0) to white (100)], a* is the green metry d/8◦. The colour parameters were determined by (−ve) to red (+ve) axis, and b* is the blue (−ve) to yel- an analytical software (UVPC Colour Analysis Personal low (+ve) axis. The parameter, C* (chroma) represents satu- Spectroscopy Software V3, Shimadzu) coupled to the ration of the colour and ho represents the hue angle. For each colorimetric parameter of the sample, measurement was made in triplicate and the average value was chosen as a result. Typically, for a given sample, the standard deviation Ba (MnO ) of the measured CIE – L*a*b* values is <0·10, and the rela- 3 4 2 tive standard deviation is not >1%. Infrared spectra were recorded in KBr pellets with a Perkin Elmer SPECTRUM 1000 spectrometer from 400–1200 cm−1. A Bruker X-band EMX spectrometer was employed to record the EPR spectra in the interval of 2000–5000 Gauss.