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Transparent single crystals of the scheelite-type  Ln [AsO 4 ] phases with  Ln  = La–Nd are obtained by the pressure-induced monazite - to-scheelite type phase transition in a Walker-type module under high-pressure and high-temperature conditions of 11 GPa at 1100–1300 °C. Coinciding with this transition, there is an increase in density and a reduction in molar volume of about 4.5 % for the scheelite-type phases (tetragonal,  I 4 1 / a ) for La[AsO 4 ] ( a  = 516.92(4),  c  = 1186.1(9) pm), Ce[AsO 4 ] ( a  = 514.60(1),  c  = 1175.44(2) pm), Pr[AsO 4 ] ( a  = 512.63(4),  c  = 1168.25(9) pm), and Nd[AsO 4 ] ( a  = 510.46(4),  c  = 1160.32(11) pm) as compared to the well-known monazite-type phases (monoclinic,  P 2 1 / n ). Surprisingly enough, the scheelite-type oxoarsenates(V) exhibit a lower coordination number for the  Ln  3+  cations ( CN  = 8 versus  CN  = 8 + 1), whereas the isolated tetrahedral [AsO 4 ] 3–  anions ( d (As–O) = 168.9–169.3 pm for the scheelites as compared to  d (As–O) = 167.1–169.9 pm for the monazites) remain almost unchanged. So the densification must occur because of the loss of two edge-connections of the involved [ Ln O 8+1 ] 15–  polyhedra with the [AsO 4 ] 3–  tetrahedra in the monazite- resulting in exclusively vertex connected [ Ln O 8 ] 13–  and [AsO 4 ] 3–  units in the scheelite-type structure.

zeitschrift für naturforschung b

High-pressure investigations of lanthanoid oxoarsenates: I. Single crystals of scheelite-type  Ln [AsO 4 ] phases with  Ln  = La–Nd from monazite-type precursors