Vibrational spectra of double diphosphates RbLnP 2 O 7 Ln  Dy, Ho, Y, Er, Tm, Yb) N. Khay * , A. Ennaciri, M. Harcharras Spectroscopy Laboratory, Department of Chemistry, Faculty of Sciences, Ibn Tofaõ Èl University, Kenitra, Morocco Received 26 March 2001; received in revised form 26 June 2001; accepted 16 July 2001 Abstract The rubidium double diphosphates RbLnP 2 O 7 Ln  Dy, Ho, Y, Er, Tm, Yb) have been conjointly studied by X-ray diffraction and vibrational spectroscopy. The infrared and Raman spectra of these compounds were established and interpreted using factor group analysis. The spectral features predicted with factor group arising from the X-ray crystallographic space group P2 1 /c C 5 2h ) for these compounds ®t the observed results. The similarities between the vibrational spectra of RbLnP 2 O 7 show that they are isostructural. Non-coincidences are observed between most infrared and Raman bands con®rm a centrosymetric structure for these compounds. The vibrational spectra point to a bent POP bridge angle in all these materials. The slight shift of the frequencies of n as POP) and n s POP) on the vibrational spectra is due to the in¯uence of the cation Ln 3 on the POP bridge angle. The POP angle values were estimated using the Lazarev's relationship. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Infrared spectra; Raman spectra; Rubidium; Rare earth; Diphosphate 1. Introduction The diphosphates M I M III P 2 O 7 involving both M I monovalent and M III trivalent cations essentially present seven different crystallographic types. The compounds of type I exhibit the KAlP 2 O 7 structure [1], whereas those of type II are isotypic with NaFeP 2 O 7 [2]. The lith- ium containing compounds form the third family and adopt the LiFeP 2 O 7 structure [3,4]. The fourth type is represented by a-NaTiP 2 O 7 [5]. A ®fth type occurs for the KYP 2 O 7 structure [6]. The sixth type concerned NaYP 2 O 7 , reveals a new structure arrangement [7]. The last type of structure NaDyP 2 O 7 [8], crystallising into the lattice of orthophosphates, called pseudo-pyropho- sphates PPP); the lattices are considered isostructural with those of the corresponding LnPO 4 . Another kind of diphosphates materials with general formula M I LnP 2 O 7 M I  Na, K, Rb, Cs; Ln  rare earth), have been deal of many studies; because some of them display interesting optical proprieties which are related to their structure [8±16]. The lanthanide compounds with potassium, rubidium and caesium belong to the type I structure, whereas the sodium compounds crystallise with the type II structure. Excepted NaLnP 2 O 7 Ln  Dy, Gd) [8,16], none of these studies have included their Raman spectra. However, no attempt of the vibrational spectroscopy of rubidium derivatives has yet been reported. In order to ®ll this gap and to elucidate the relation- ship between spectra and structure, we have carried out the study of vibrational spectra of RbLnP 2 O 7 Ln  Dy, Ho, Y, Er, Tm, Yb). Vibrational Spectroscopy 27 2001) 119±126 * Corresponding author. Tel.: 212-6166-2986; fax: 212-2282-6361. E-mail address: khay.noureddine@caramail.com N. Khay). 0924-2031/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0924-203101)00123-0