Neutron diffraction reveals the existence of confined water in triangular and hexagonal channels of modified YPO4 at elevated temperatures

Abstract

We provide experimental evidence for confinement of water molecules in the pores of hexagonal structure of YPO4 at elevated temperatures upto 600 K using powder neutron diffraction. In order to avoid the large incoherent scattering from the hydrogen, deuterated samples of doped YPO4:Ce-Eu were used for diffraction measurements. The presence of water molecules in the triangular and hexagonal pores in the hexagonal structure was established by detailed simulation of the diffraction pattern and Rietveld refinement of the experimental data. It was observed that the presence of water leads specifically to suppression of the intensity of a peak around Q = 1.04 Å -1 while the intensity of peaks around Q=1.83Å -1 is enhanced in the neutron diffraction pattern. We estimate the number of water molecules as 2.36 (6) per formula units at 300 K and the sizes of the hexagonal and triangular pores as7.2 (1) Å and 4.5 (1) Å, respectively. With increase in temperature, the water content in both the pores decreases above 450 K and vanishes around 600 K. Analysis of the powder diffraction data reveals that the hexagonal structure with the pores persist up to 1273 K, and transforms to another structure at 1323 K. The high temperature phase is not found to have the zircon or the monazite type structure, but a monoclinic structure (space group P2/m) with lattice parameters am= 6.826 (4) Å, bm= 6.645 (4) Å, cm= 10.435 (9)Å, and β= 107.21 (6)°. The monoclinic structure has about 14 % smaller volume than the hexagonal structure which essentially reflects the collapse of the pores. The phase transition and the change in the volume are also confirmed by x-ray diffraction measurements. The hexagonal to the monoclinic phase transition is found to be irreversible on cooling to room temperature. 2 IntroductionRare-earth (RE) phosphates and its derivatives (REPO4:RE=La, Ce, Gd or Y) exhibit exotic properties like chemical stability, excellent high-temperature properties, high radiation damage tolerance, high luminescence quantum yield, high energy band gap, etc [1][2][3][4][5][6][7][8][9][10][11][12] . Due to a sharp emission, these compounds are also used as host for luminescence applications 6,7,13 including lasers and display devices. A combination of rare-earth phosphates and silicates is used as host for nuclear wastes 14 . At ambient condition, depending on the ionic radii of RE ion and synthesis route, the compounds can crystalize in tetragonal, hexagonal and monoclinic phases 13,[15][16][17][18] .A smaller ionic radius compared to the ionic radius of Gd generally adopts the tetragonal structure, whereas other orthophosphates have the lower-symmetry monoclinic structure. Depending on the synthesis conditions, GdPO4, TbPO4, DyPO4, and HoPO4 can adopt either zircon or monazite structure 13,16,[19][20][21] . LaPO4 crystallizes in two polymorphs, namely, the tetragonal phase (xenotime structure) and the monoclinic phase (monazite structure). Yttrium orthophosphate, YPO4, also has the tetragonal symmetry (xenotime type) 6 . Doping YPO4 matri...