Orthorhombic Pnma Phase Research Articles

Neutron powder structural studies of UF 6, MoF 6 and WF 6 at 77 K

The crystal structures of the hexafluorides UF 6, MoF 6 and WF 6 have been studied at 77 K by powder neutron diffraction. Profile refinements gave R = ∑(ϵy oϵϵy cϵ)/∑y o values of 6.9, 7.6 and 5.8% respectively, where Y o and Y c are the background — corrected pattern intensities. The structures are compared with the room-temperature structure of UF 6, the structures of MoF 6, WF 6 at 193K and with the plastic cubic structures of MoF 6, WF 6 and SF 6. The metal-fluorine bond lengths do not decrease significantly on cooling to 77 K but the octahedra pack more efficiently, and the atomic coordinates more closely approach the ideal coordinates based on perfect hexagonal close-packing of the fluorine atoms. Because of their shorter metal-fluorine bonds, the MoF 6 and WF 6 structures are more distorted from the ideal than UF 6. MoF 6, WF 6 and especially SF 6 are more compact and spherically-shaped molecules than UF 6; this explains the absence of a plastic cubic phase for UF 6, and the greater stability range of the SF 6 cubic phase. The existence of the orthorhombic Pnma phase for UF 6, MoF 6 and WF 6 down to 77 K is confirmed.

Correlations between site geometries and level energies in the laser system Nd1−x Y x P5O14

The pentaphosphate system Nd1−x Y x P5O14 covers four crystalline phases with five different site geometries for Nd. We determined the4F3/2,4I13/2,4I11/2,4I9/2 stark levels and identified the sites spectroscopically, including the case of the monoclinic layer structureC2/c which has two Nd sites. Crystal field calculations for the energy levels in the monoclinic phaseP2 1/c and the orthorhombic phasepnma with tetragonal and orthorhombic site symmetry approximations are reasonable but reveal their shortcomings. As a consequence, we consider it improper to try to calculate rare-earth optical transition probabilities with approximate crystal fields. Random distribution of Nd and Y causes inhomogeneous broadening (∼6 A) which peaks atx=0.5. Thermal equilibrium between the4F3/2 levels is reached within 2–5×10−13 sec. The three room temperature modifications could be lased cw. Pure NdP5O14 was found to be the best laser.