JOURNAL OF MATERIALS SCIENCE LETTERS 9 (1990) 235-236 Hydrothermal synthesis and structure of TmPsO14 K. BYRAPPA, S. SRIKANTASWAMY The Mineralogical Institute, University of Mysore, Manasagangotri, Mysore 570 006, India S. GALl Departament de Cristallografia, Universitat de Barcelona, Marti i Franquos, sn 08028, Barcelona, Catalonia, Spain Rare earth ultraphosphates attracted great interest during the 1970s because of their spectroscopic properties. Although hundreds of reports appeared on various aspects of these ultraphosphates, there are only three reports on the synthesis of RP5014 by the hydrothermal method [1-3]. Similarly, there is only one report on the growth of alkali rare earth poly- phosphates by the hydrothermal technique [4]. This is mainly because of the difficulties associated with the hydrothermal method, particularly in the phosphoric acid media. However, the growth of rare earth ortho- phosphates by any method is quite interesting because of its structural peculiarities. In the earlier reports of the growth of NdP5Ol4 or MNdP4OI2, the growth was carried out at elevated temperature in order to elimi- nate the possibility of (OH)- molecules entering the growing crystal [1-3]. Further, the size of the NdPsO~4 crystals obtained by the hydrothermal technique was only 0.1 to 0.3mm at 650°C [1]. Here, we report the synthesis of TmPsO~4 by the hydrothermal technique, and present the results of the X-ray diffraction studies. The synthesis of TmPsO~4 was carried out using Morey type autoclaves with teflon liners of capacity 50 ml, under low pressure and temperature conditions (240°C and 100 atm). The starting materials, such as Tm203 and 85% orthophosphoric acid were taken in the molar ratio 1 : 5 in a teflon liner. Some admixtures like A13+ and As 3+ in very small amounts were added to the nutrient. The autoclave assembly was then placed in the crystallization chamber and the tem- perature was slowly increased (at the rate of 10° C h-~ ) up to 240 ° C and held for seven days. The per cent fill in the liner was about 60. The crystallization was carried out through spontaneous nucleation. After the experimental run, the autoclave was quenched sud- denly and the resultant product was washed in hot water to remove the excess phosphoric acid. Well- developed crystals of TmPsO~4 belonging to the mono- clinic system with crystal size varying from 0.5 to 5 mm were collected from the teflon liner. The crystals show well-developed prismatic structure with forms 00 1 and 1 00, and less developed 1 1 0 and - 1 1 1, buff white colour and vitreous lustre. The absence of (OH) was confirmed through infrared spectroscopy. Thus, the present study shows that it is possible to grow good quality rare earth ultraphosphates at rela- tively low pressure and temperature conditions without the presence of (OH)- molecules in the final product, provided the suitable molar ratio of R203 and P205 is selected. The addition of impurities like A13+ and As 3+ have only contributed to the growth of larger crystals by spontaneous crystallization and they have not entered the resultant product in signifi- cant amounts. The present study also indicates that it is possible to obtain different polymorphic modifi- cations of RP5014 by selecting suitable pressure and temperature conditions. A crystal about 0.2 x 0.2 x 0.2mm in size was selected to obtain X-ray diffraction intensities in a single crystal Enraf-Nonius CAD-4 diffractometer, with radiation 2(MoK~) = 0.07107nm. A total of 3164 reflections were measured in a triclinic lattice. The analysis of these data revealed that the structure should be monoclinic, space group Cc or C2/c, with cell parameters given in Table I. These parameters are almost identical to these reported previously for HoPsO14 [5] and ErPsOl4 [6]. At this stage an isotropic structure refinement was carried out starting with the atomic parameters given by [6]. Final results showed that TmPsO~4 is completely isostructural with both HoPsOj4 and ErPsOI4. Slight differences in cell volume are due to decreasing ionic radius of the trivalent rare earth cations (Table I). The space group was then unambiguously determined as C2/c. Up to now, three different RP5014 structure types have been reported: (i) P2~/c in NdPsOl4 [7]; (ii) Pnma, in HoPsOi4 [8]; and (iii) C2/c inthe compounds given above and listed in Table I. Types (i) and (ii) are very similar, while type (iii) is different, consisting of (PO3)oo chains along [00 1] linked by PO4 tetrahedra. Three fifths of the PO 4 tetrahedra have two free oxy- gen atoms and the remaining two fifths have one. In all three structure types, each rare earth atom has eight oxygen neighbours, and no oxygen is shared with any other RO 8 polyhedron. T A BLE I Cell parameters for C2/c structure type RPsOI4 compounds Compound a b c (nm) fl (°) V c (nm 3) Z Ionic radius (nm) HoPsO 14 1.2881 (5) 1.2771 (5) 1.2424(5) 91.34(5) 2.0432 8 0.097 ErPsOI4 1.2837(8) 1.2723(7) 1.2381(7) 91.25(5) 2.021 6 8 0.096 TmPsO~4 1.2817(4) 1.2719(4) 1.2351(1) 91.25(2) 2.0129 8 0.095 0261-8028/90 $03.00 + .12 © 1990 Chapman and Hall Ltd. 235