Structure of potassium germanophosphate glasses by X-ray and neutron diffraction. Part 1: Short-range order U. Hoppe a, * , R.K. Brow b , N.P. Wyckoff b , A. Schöps c , A.C. Hannon d a Institut für Physik, Universität Rostock, 18051 Rostock, Germany b Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA c Hamburger Synchrotronstrahlungslabor HASYLAB am Deutschen Elektronen-Synchrotron DESY, 22607 Hamburg, Germany d ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom article info Article history: Received 8 July 2007 Received in revised form 4 February 2008 Available online 15 April 2008 PACS: 61.10.Nz 61.12.Ld 81.05.Kf Keywords: Neutron diffraction/scattering X-ray diffraction Oxide glasses Germanates Phosphates Short-range order abstract It has been reported that the addition of K 2 O or P 2 O 5 to binary germanate glasses increases the GeO coordination numbers (N GeO ). The present work describes X-ray and neutron diffraction studies aimed at clarifying the concomitant effects of both oxides on the structures of ternary K 2 O–GeO 2 –P 2 O 5 glasses. The GeO coordination numbers obtained range from 4.2 to 5.1, less than what is predicted according to a model which assumes all oxygen atoms form network bridges similar to those found in the related crys- tal structures. This implies that the glass structures must include terminal oxygen sites, likely associated with the PO 4 tetrahedra, that are neutralized by coordinating K + ions. The shapes of the high resolution first-neighbor diffraction peaks do not indicate distinctly different species of PO and GeO bonds. The model for the increase of N GeO which is based on an increase of the fraction of GeO 6 units, at the expense of GeO 4 units, is supported by the analysis of the two main components of the GeO peak used in the fits. However, the existence of a GeO 5 fraction cannot be excluded from the present data sets. A linear relation between the total GeO coordination numbers and mean GeO distances exists, assuming end members of units GeO 4 and GeO 6 with bond lengths of 0.175 and 0.190 nm, respectively. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The structures of binary germanophosphate glasses (1  x)- P 2 O 5 –xGeO 2 have been studied by diffraction methods and an increase of the GeO coordination numbers, N GeO , with increasing P 2 O 5 content was detected [1–4]. The recent experiments with greater resolving power [3,4] have found that N GeO depends on the fraction of GeO 2 according to N GeO ¼ 4 þ 2ð1  xÞ=x: ð1Þ This dependence implies that all oxygen atoms are bridging ones (O B ), forming a continuous network with GeOGe, POGe and/or POP links. The latter oxygen sites are expected only with sufficient P 2 O 5 additions; i.e., P 2 O 5 > 40 mol%. N GeO values less than predicted by Eq. (1) imply the existence of terminal oxygens (O T ) such as present in the PO 4 tetrahedra of vitreous P 2 O 5 [5]. N GeO val- ues greater than predicted by Eq. (1) require threefold connected oxygens such as present in high-pressure densified GeO 2 glass [6] or the rutile-type GeO 2 crystal [7]. 17 O magic angle spinning (MAS) NMR analyses of binary germanophosphate glasses indicate that only O B oxygens are present in the glasses studied with 0.75 6 x 6 1.0 [4]. Binary germanophosphate glasses with N GeO val- ues 6 have not been realized [1–4] because of difficulties prepar- ing binary glasses of P 2 O 5 contents which approach those of the crystals Ge 5 O(PO 4 ) 6 [8] (x = 0.625) and GeP 2 O 7 [9] (x = 0.5). Ternary A 2 O–GeO 2 –P 2 O 5 (A – alkali atom) glasses with large fractions of GeO 6 octahedra should be possible if one considers that fractions of SiO 6 octahedra are detected in Na 2 O–SiO 2 –P 2 O 5 glasses by 29 Si MAS NMR [10]. For small SiO 2 contents (<0.10) and large P 2 O 5 contents (>0.55), all Si atoms occupy octahedral sites [11]. The structures of the KGeOPO 4 [12] and KGe 2 (PO 4 ) 3 [13] crystals possess only GeO 6 octahedra, and similar structures could be pos- sible for glasses with equivalent compositions. Several series of ter- nary A 2 O–GeO 2 –P 2 O 5 glasses have already been investigated by Raman spectroscopy [14–16]. A glass series along the GeO 2 –NaPO 3 join [14] revealed a change from NaPO 3 –GeO 2 orthophosphate structures via 4NaPO 3 –GeO 2 pyrophosphate structures to NaPO 3 metaphosphate structures which means the PO 4 tetrahedra partic- ipate in 0, 1 and 2 POP bridges, respectively. It was stated that 0022-3093/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2008.03.012 * Corresponding author. Tel.: +49 381 4986844; fax: +49 381 4986862. E-mail address: hoppe@physik1.uni-rostock.de (U. Hoppe). Journal of Non-Crystalline Solids 354 (2008) 3572–3579 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol