A neutron and X-ray diffraction study of the structure of the LaP3O9 glass

https://doi.org/10.1016/S0022-3093(98)00396-2Get rights and content

Abstract

A study of La metaphosphate glass using a combination of neutron and X-ray diffraction has yielded reliable information on the La–O coordination and the spatial distribution of the La3+ cations. Due to the real-space resolution of the neutron diffraction experiment, it was possible to observe two different P–O bond lengths in the PO4 tetrahedron. The La–O coordination number of about 7 exceeds the number, 6, of terminal oxygen atoms available for the coordination of each of the La3+ cations. This leads to a tendency for clustering of the LaOn polyhedra, which is also observed in complementary reverse Monte Carlo results. The first feature visible in the X-ray structure factor at a scattering vector magnitude, Q, of 12 nm−1, is related to a La–La peak in real space at about 640 pm. This corresponds to La–La neighbours which are separated by a PO4 unit.

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 5D07F0 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.

References (27)

  • K. Hirao et al.

    J. Non-Cryst. Solids

    (1994)
  • R.K. Brow et al.

    J. Non-Cryst. Solids

    (1994)
  • U. Hoppe et al.

    J. Non-Cryst. Solids

    (1995)
  • U. Hoppe

    J. Non-Cryst. Solids

    (1996)
  • M. Rzaigui et al.

    J. Solid State Chem.

    (1984)
  • J. Matuszewski et al.

    J. Solid State Chem.

    (1988)
  • A.J. Leadbetter et al.

    J. Non-Cryst. Solids

    (1972)
  • S.R. Elliott

    J. Non-Cryst. Solids

    (1995)
  • M.J. Weber, in: J. Zarzycki (Ed.), Materials Science and Technology, A Comprehensive Treatment, vol. 9, VCH, Weinheim,...
  • U. Hoppe et al.

    Z. Naturforsch. A

    (1995)
  • U. Hoppe et al.

    Z. Naturforsch. A

    (1996)
  • C. Jäger, unpublished...
  • H.Y.P. Hong

    Acta Crystallogr. B

    (1974)
  • Cited by (57)

    • Structural and thermal properties of La <inf>2</inf>O <inf>3</inf>-Fe <inf>2</inf>O <inf>3</inf>-P <inf>2</inf>O <inf>5</inf> glasses

      2012, Journal of Molecular Structure
      Citation Excerpt :

      Previous studies have shown iron atom appears in a four- and six-fold coordination environment [5,17]. However, the lanthanum atom coordination number is greater than six [18]. The higher coordination number can lead to a strong polymerization effect easier.

    View all citing articles on Scopus
    View full text