A neutron and X-ray diffraction study of the structure of the LaP3O9 glass
Introduction
A study of the structural effect of the incorporation of rare-earth ions into phosphate glasses is interesting, because such materials possess a potential for applications in laser techniques and optoelectronics [1]. Most of these glasses contain the rare earth as a minor dopant but, in such systems, structural details are difficult to investigate by diffraction methods. Some details of the cationic environment in host glasses were estimated by comparing the line widths of the 5D0 → 7F0 transition of Eu3+ in several oxide glasses. Hirao et al. [2]have found that phosphate systems yield the most flexible oxygen environments for the modifier cations. Here this will be examined using a combination of neutron diffraction (ND) and X-ray diffraction (XRD) experiments. For simplicity, and as a continuation of previous work 3, 4, a binary La metaphosphate glass was chosen as the sample.
Some general properties of phosphate glasses are well known: The metaphosphate systems are formed from 2-connected PO4 tetrahedra (Q2 units). Even for an AlP3O9 glass, this structure was shown by 31P magic angle spinning nuclear magnetic resonance (MAS–NMR) studies [5]. Both terminal oxygen atoms (OT) of the Q2 unit behave similarly, as indicated by Raman spectroscopy and O1s X-ray photoelectron spectroscopy (XPS) data [6]. Owing to the trend in the packing densities and Me–O coordination numbers, NMeO, for MeO–P2O5 glasses, with Me=Zn, Mg, Ca, and Ba, it was suggested [7]that all of the OT atoms prefer Me–OT–P bridging positions, while the bridging oxygen atoms (OB) are found as P–OB–P bridges. Thus, the ratio MTO=n(OT)/n(Me) and the Me–O coordination number are critical for the formation of the glass structure [8], where MTO is equal to v(y+1)/y; v is the valency of Me and y is given by n(Me2/vO)/n(P2O5). For the case when NMeO=MTO, the type of phosphate glass formed is denoted type II in Ref. [8]. In such networks, all of the oxygen atoms are found in bridging positions. Thereby, the oxygen polyhedra of the modifier cations do not share common vertices. If v=3, this type of structure is expected to be formed in glasses with y=0.6 and NMeO=8 and, in metaphosphate glasses (y=1), with NMeO=6. This structure is consistent with the crystalline structures of CeP5O14 [9], YbP3O9 [10], and AlP3O9 [11]and, in addition, NAlO is found to be ≅6 in an AlP3O9 glass [3]. This number of NMeO is also expected in metaphosphate glasses for the smaller rare-earth ions, but not for the larger La3+ ions. In crystalline LaP3O9 [12], the La3+ ions are accommodated in distorted LaO8 polyhedra, which share edges with two adjacent LaO8 units. An investigation of the arrangement of the La-centered oxygen polyhedra in LaP3O9 glass is the aim of this work, in which information concerning the overall structure has been extracted by reverse Monte Carlo (RMC) simulations.
Previously, other authors 13, 14, 15have studied the Ln–O coordination for various lanthanide (Ln) ions in binary metaphosphate glasses. The Ln–O distances observed by extended X-ray absorption fine structure (EXAFS) measurements have confirmed the so-called lanthanide contraction [13]. The uncertainties in the NLnO coordination numbers do not allow any conclusion to be reached concerning a trend from 8-fold to 6-fold coordination as found in the related crystals. Only a general estimation of NLnO as being in the range from 6 to 8 is given. Neither these EXAFS measurements nor published XRD results 14, 15show any evidence for a mutual ordering of the modifier ions.
Section snippets
Sample preparation
The sample was prepared in a platinum crucible, starting from crystalline LaP3O9. The liquid was held at a temperature of 1560°C for 1 h and then poured into a mould which, subsequently, was held in an annealing furnace at 600°C for 0.5 h. A compositional analysis of the glass (y=1.016 ± 0.008) revealed a slight excess of La2O3, appropriate for the metaphosphate composition. The mass density of the glass was 3.223 ± 0.006 g cm−3.
Diffraction experiments
The neutron diffraction experiments were performed using the
Results
The parameters for the nearest neighbour peaks in the real-space correlation function, T(r), were obtained using peak fitting techniques. Both T(r) functions are shown in Fig. 2. The neutron curve was obtained by Fourier transformation of SN(Q) up to Qmax=470 nm−1 without using any modification function. The value of Qmax used for the X-ray data was 150 nm−1 and a Lorch modification function was applied. The effect of truncation on the Fourier transformation was taken into account in the fits,
Discussion
The occurrence of equal fractions for the two P–O distances proves that n(OT)/n(OB)=2, as determined by O1s XPS in metaphosphate systems [6]. The electric field strength of the modifier cations has a definite effect on the P–OT and P–OB bond lengths 3, 4. The P–O peak for the LaP3O9 sample is split to an extent comparable to that observed for the modifier cations Zn2+, Mg2+, and Pb2+. The P–O peak appears to be more split than for an AlP3O9 glass [3], but less than for KPO3 [4].
The La–O
Conclusions
Despite some shortcomings in the details of the models generated by the RMC method, reliable information concerning the arrangement of the La3+ modifier cations in LaP3O9 glass was obtained. A distance peak in gLaLa(r) at about 640 pm is related to La–La first neighbours separated by a common PO4 neighbour. The tendency for clustering of the LaOn polyhedra is consistent with the La–O coordination number of about 7. Most of the OT atoms occupy La–OT–P bridging sites. This feature enables the
Acknowledgements
The financial support of the BMBF (Grant 03-KR4ROK-1) is gratefully acknowledged.
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