Abstract
Alkali rare-earth double phosphates have been studied for use as long-wavelength scintillators for γ-ray detection using Si photodiodes. Single-crystal and powder X-ray diffraction (XRD) and powder neutron diffraction have been used to study the structure as a function of temperature. K 3Lu(PO4)2 crystallizes with a hexagonal unit cell at room temperature, space group P3̄. The Lu ion is six-coordinated to the oxygen atoms of the phosphate groups. Two lower-temperature phases were characterized using single-crystal XRD and powder neutron diffraction. The first transition occurs at 230 K with a transformation to a monoclinic P2 1/m space group symmetry, and the Lu retains six coordination. The second phase transition occurs at 130 K, with a large change in the cell volume, keeping the same P21/m space group symmetry; however, one of the phosphate groups rotates to increase the coordination of the Lu ion to seven. This is an unusual example of an isosymmetric phase transition with a coordination change, driven by temperature. High-temperature powder neutron diffraction and high-temperature powder XRD have been used to study the thermal expansion of K3Lu(PO4)2 and indicate a large thermal expansion anisotropy. The crystallographic axes with largest changes account for the structural collapse, which rotates the phosphate group to increase the Lu coordination. The lowest temperature form of K 3Lu(PO4)2 is the same as the room temperature form for all the lighter RE compounds of the same type, which is not surprising, given the lighter (larger) RE ions would prefer a higher coordination number.
Original language | English |
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Pages (from-to) | 182-189 |
Number of pages | 8 |
Journal | Journal of Alloys and Compounds |
Volume | 588 |
DOIs | |
State | Published - Mar 5 2014 |
Funding
Research conducted at ORNL’s High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Research sponsored by the Office of Nonproliferation Research and Engineering in the Office of Defense Nuclear Nonproliferation in the National Nuclear Security Administration. Research at the Oak Ridge National Laboratory for one author (LAB) is sponsored in part by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. Research at LSU is supported by NSF DMR-1002622 (RJ).
Keywords
- Crystal structure
- Phase transitions
- Phosphate
- Powder neutron diffraction
- Scintillator
- Single-crystal X-ray diffraction