An extended x-ray absorption fine structure study of rare-earth phosphate glasses near the metaphosphate composition
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An extended x-ray absorption fine structure study of rare-earth phosphate glasses near the metaphosphate composition R. Anderson School of Physical Sciences, The University of Kent, Canterbury, CT2 7NR, United Kingdom
T. Brennan School of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, United Kingdom
J.M. Colea), G. Mountjoy, D.M. Pickup, and R.J. Newport School of Physical Sciences, The University of Kent, Canterbury, CT2 7NR, United Kingdom
G.A. Saunders School of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, United Kingdom (Received 17 November 1998; accepted 23 September 1999)
A variable-temperature (79, 145, and 293 K) extended x-ray absorption fine structure study, using rare-earth LIII absorption edges, is reported for phosphate glasses doped with rare-earth elements (R, where R ⳱ La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) with compositions close to metaphosphate, R(PO3)3. The results yield nearest-neighbor R–O distances that demonstrate the lanthanide contraction in a glassy matrix and an R–O coordination intermediate between 6 and 7 for rare-earth ions with smaller atomic number (Z) and 6 for rare-earth ions with larger Z. Thermal parameters show no significant changes in R–O distances or coordination numbers between 293 and 79 K. There is evidence of an R–P correlation between 3.3 and 3.6 Å and the beginning of a second R–O correlation at approximately 4 Å. No R–R correlations up to a distance of approximately 4 Å were observed.
I. INTRODUCTION
Rare-earth-doped phosphate glasses with compositions close to metaphosphate R(PO3)3 contain very large concentrations of rare-earth ions and as a consequence exhibit novel and useful magnetic and magneto-optical properties. The structural disorder, characteristic of amorphous materials, results in the universal feature of a wide distribution of low-energy excitations that dominate their thermal and optical properties. Recently, this characteristic feature of the glassy state has been extended to magnetic states.1,2 Measurements of the low-temperature specific heat and low-frequency Raman scattering have demonstrated the existence of large magnetic contributions to rare-earth metaphosphate glasses. In the case of the Pr glass, multiple splitting of electronic transitions between 4f levels due to topological disorder produces a wide spectrum of low-lying magnetic excitations. The
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Address all correspondence to this author. Present address: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom. e-mail: [email protected]
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J. Mater. Res., Vol. 14, No. 12, Dec 1999 Downloaded: 15 Mar 2015
magnetic contributions to the low-temperature specific heat and low-frequency Raman scattering of the Gd and Sm glasses are quite different in kind from those found for the Pr glass. Understanding the plethora of diverse and unusual properties shown by these magnetic glasses requires knowledge of their structures on the
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