ESR Study of the Monoclinic—Orthorhombic Phase Transition in Neodymium Pentaphosphates

Very accurate electron spin resonance measurements are performed on Gd 3+ impurities in single‐domain crystals of neodymium pentaphosphates. It is shown that the transition from C s symmetry above the critical temperature ( T c = 421 K) to C 1 symmetry below T c can be interpreted in terms of a temperature‐dependent three‐dimensional rotation R(α, β, γ) of the principal axes of the tensor D . The Euler's angle β( T ) and (α + γ) ( T ) are given by β(degrees) = 0.415 ( T c − T ) 1/2 − 0.007 ( T c − T ) and (α + γ) (degrees) = 0.11 ( T c − T ) 1/2 . Finally, the temperature‐dependent rotation of the tensor D is compared to the rotation of the quadrupolar tensor (tensor Q ) appearing in the multipolar expansion of the internal crystal field.