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We provide experimental evidence for confinement of water molecules in thepores of hexagonal structure of YPO4 at elevated temperatures upto 600 K usingpowder neutron diffraction. In order to avoid the large incoherent scatteringfrom the hydrogen, deuterated samples of doped YPO4:Ce-Eu were used fordiffraction measurements. The presence of water molecules in the triangular andhexagonal pores in the hexagonal structure was established by detailedsimulation of the diffraction pattern and Rietveld refinement of theexperimental data. It was observed that the presence of water leadsspecifically to suppression of the intensity of a peak around Q = 1.04{\AA}-1while the intensity of peaks around Q=1.83{\AA}-1 is enhanced in theneutron 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 triangularpores as7.2 (1) {\AA} and 4.5 (1) {\AA}, respectively. With increase intemperature, the water content in both the pores decreases above 450 K andvanishes around 600 K. Analysis of the powder diffraction data reveals that thehexagonal structure with the pores persist up to 1273 K, and transforms toanother structure at 1323 K. The high temperature phase is not found to havethe zircon or the monazite type structure, but a monoclinic structure (spacegroup P2/m) with lattice parameters am= 6.826 (4) {\AA}, bm= 6.645 (4) {\AA},cm= 10.435 (9){\AA}, and \b{eta}= 107.21 (6){\deg}. The monoclinic structurehas about 14 % smaller volume than the hexagonal structure which essentiallyreflects the collapse of the pores. The phase transition and the change in thevolume are also confirmed by x-ray diffraction measurements. The hexagonal tothe monoclinic phase transition is found to be irreversible on cooling to roomtemperature.

Neutron Diffraction Reveals the Existence of Confined Water in Triangular and Hexagonal Channels of Modified YPO4 at Elevated Temperatures