Vibrational and optical properties of rare earth metaphosphate glasses

Phosphate glasses containing rare earth ions have important apllications in laser and optoelectronics technology. A wide range of rare earth metaphosphate glasses (REMG) of the type $(R_2O_3)_0_,_2_5(P_2O_5)_0_,_7_5$, where R represents one of the rare earth elements Ce, Pr, Sm, Nd, Eu, Gd, Tb, Dy, Ho, and Er or La or Y, and the mole fraction x is close to 0,25, have been made. Employment of the complementary structural probes EXAFS and X-ray diffraction has shown that these glasses have a network built up from $PO_4$ tetrahedral units, confirming indications from Raman spectra. Systematic studies of the elastic, anelastic, and non-linear acoustic properties, usign ultrasonic techniques, Raman, optical absorption spectra, laser induced fluorescence and specific heat, involving intemational collaboration between groups at the Universities of Bath, Messina, Trento, Southampton, Edinburgh, Kent, and the Witwatersrand, are reviewed here. These REMG display a plethora of unusual physical properties. Ultrasonic wave velocity measurements under pressure show that the acoustic modes, especially the longitudinal ones, of the Sm and Eu phosphate glasses soften under pressure: these glasses become easier to squeeze when subjected to high pressure, an extraordinary effect. For a $(Sm_20_3)_0_,_2_2(p_20_5)_0_,_7_8$ glass the thermal expansion is small and below 110KL becomes negative - at low temperatures the contributions from the soft long wavelenght acoustic modes dominate. The REMG show the excess specific heat at low temperature characteristic of a glass. The vibrational modes responsible also influence the low frequency Raman spectra: both a "boson peak" and the strongly temperature dependent, light scattering excess (LSE) are observed. The ultrasonic attenuation shows a plateau below 15K, due to pho-non-assisted tunnelling in two-level system, and at heigher temperatures rises to the broad peak typical of oxide glasses, whose maximum shifts to heigher temperatures as the ultrasonic driving frequency is raised. An important question under discussion is whether the physical properties of these REMG can be accounted for within the framework of recent theoretical advances, in particular the Soft Potential Model (SPM), which in principle could provide a unified explanation of the universal complex of their anomalous thermal, acoustic and optical vibrational properties