Journal of Solid State Chemistry 174 (2003) 44–51 Synthesis, electron diffraction, XRD and DSC study of the new elpasolite-related oxyfluoride, Tl 3 MoO 3 F 3 F.J. Brink,  L. Nore´n, and R.L. Withers Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia Received 22 November 2002; received in revised form 10 March 2003; accepted 15 March 2003 Abstract Tl 3 MoO 3 F 3 ,apreviouslyunreportedmemberofthe A 2 BM VI O 3 F 3 familyofelpasolite-relatedoxyfluorides,hasbeenpreparedby the reaction of TlF with MoO 3 at 655  C. DSC shows two major polymorphic phase transitions at 42  C and 130  C, respectively. Electron diffraction and XRD studies of the complex room temperature polymorphic form of this material indicate that the spacegroup symmetry is monoclinic P1a1 with a superstructure unit cell given by a ¼ 3a p þ c p ; b ¼ 3b p ; c ¼ 1 2 ða p þ 3c p Þ; when expressedwithrespecttotheunderlyingidealelpasolite-typeparentstructure.Thissuperstructure,whilerelated,isnotisomorphous tothatrecentlyreportedforK 3 MoO 3 F 3 .Theexistenceofasharedsubsetofstrong G p 7J =5½204  p type satellite reflections suggests the existence of a common intermediate superstructure. A highly structured, three-dimensional continuous diffuse intensity distribution is observed in Tl 3 MoO 3 F 3 and Rb 2 KMoO 3 F 3 . This suggests that a particular pattern of local O/F ordering and associatedMoionshifts,recentlyshowntoberesponsiblefortheexistenceofthisdiffusedistributioninthecaseofK 3 MoO 3 F 3 ,may be common to the entire family of elpasolite-related A 2 BM VI O 3 F 3 compounds. r 2003 Elsevier Science (USA). All rights reserved. 1. Introduction There exists a relatively large number of oxyfluoride phases(A 2 + B + M VI O 3 F 3 ; A, B=alkali metal; M VI =Mo, W), which crystallize either in the ideal cubic ðFm % 3mÞ A 2 BMX 6 elpasolite structure type (see Fig. 1) or in closely related displacive variants thereof [1–10]. To date, such elpasolite-related oxyfluoride phases have beenfoundforalkalielementsranginginsizealltheway from Na + to Cs + . The ideal cubic A 2 BMX 6 elpasolite, or parent, structure type (see Fig. 1) usually occurs in such materials only as a high temperature polymorphic form (the g-polymorph). On cooling, this high tempera- ture g-polymorph typically undergoes two reversible structural phase transitions (at temperature T 2 to a b- polymorph and at temperature T 1 to a lowest tempera- ture a-polymorph) [2–10]. Both these lower temperature polymorphic forms have been reported to be simulta- neously ferroelectric as well as ferroelastic. As a result, theopticalanddielectricpropertiesofthematerialshave been extensively investigated [2–10]. Their rather com- plex crystallography, however, is far less well under- stood [2–7,11,12]. When the alkali elements are K, Rb or Cs (A; B=K, Rb,Cs),thefirstphasetransitionattemperature T 2 has been attributed to a spontaneous ferroelastic distortion of the metric symmetry of the parent structure from cubic to tetragonal [1–4]. The subsequent second phase transitiontothe a-polymorph, on cooling below T 1 ; has been reported to be correlated with the appearance of additional satellite reflections, i.e., to the onset of a superstructure [2–4,11,12]. The unit cell and symmetry of this superstructure (typically the room temperature polymorph) has in the past been the subject of considerable speculation as it has been variously described as tetragonal, monoclinic, trigonal as well as triclinic, based on studies of Rb 2 KMoO 3 F 3 and K 3 MoO 3 F 3 [2–7]. A very recent electron diffraction study of the complex a-polymorph of K 3 MoO 3 F 3 [11] has resolved many of these inconsistencies and deter- mined the symmetry, of at least this polymorph, to be ARTICLE IN PRESS  Correspondingauthor.ElectronMicroscopeUnit,ResearchSchool ofBiologicalSciences,AustralianNationalUniversity,CanberraACT 0200, Australia. Fax: +61-2-6125-3218. E-mail address: brink@rsbs.anu.edu.au (F.J. Brink). 0022-4596/03/$-see front matter r 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0022-4596(03)00172-5