Hydrothermal synthesis of lithium zinc phosphates:: Structural investigation of twinned α-Li4Zn(PO4)2 and a high temperature polymorph β-Li4Zn(PO4)2

The system LiOH:Zn(NO3)(2):H3PO4:H2O was investigated using hydrothermal synthesis techniques in the temperature range 20degreesC < T < 602degreesC and pressure range, 1 < p < 2.8 kbar, and the crystallization fields of some lithium zinc orthophosphates are reported. Small crystals of a twinned lithium zinc phosphate, alpha-Li(4)ZH(PO4)(2) were prepared and a structural investigation was performed using synchrotron X-ray diffraction: monoclinic, space group P2(1) /a, a = 8.096(2) Angstrom, b = 10.242(2) Angstrom, c = 8.106(2) Angstrom, beta = 104.73(3)degrees, V = 650.1(2) Angstrom(3) and Z = 4. The refinement gave R(F) = 0.042 and wR(F-2) = 0.068 based on 968 independent reflections and 616 observed, F-o(2) > 3sigma(F-o(2)) (lambda = 0.9346(1) Angstrom). The cations are tetrahedrally coordinated to oxygen, and the structure can be viewed as layers of regular ZnO4 and PO4 tetrahedra interconnected by more distorted LiO4 tetrahedra. Thermal investigation using in situ powder X-ray diffraction in the temperature range 20degreesC < T < 900degreesC was performed. The reversible phase transition alpha --> Li4Zn(PO4)(2) was observed in agreement with previous investigations. A powder pattern measured at T = 900degreesC was indexed with a triclinic unit cell: space group, P (1) over bar, a = 5.173(3) Angstrom, b = 7.97(1) Angstrom, c = 9.92(1) Angstrom, alpha = 80.9(1)degrees, beta = 126.8(1)degrees and gamma = 106.0(1)degrees, V = 319.8(7) Angstrom(3) and Z = 2. The linear thermal expansion of alpha- and beta-Li4Zn(PO4)(2) are extracted from powder diffraction data in the temperature range RT to 900degreesC. (C) 2002 Elsevier Science (USA).