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

The pentaphosphate system Nd_1?x Y _x P₅O₁₄ covers four crystalline phases with five different site geometries for Nd. We determined the⁴F_3/2,⁴I_13/2,⁴I_11/2,⁴I_9/2 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 Å) which peaks atx=0.5. Thermal equilibrium between the⁴F_3/2 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.