Journal of Alloys and Compounds 494 (2010) 336–339 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Stability field and structural properties of intra-rare earth perovskites C. Artini a,∗ , G.A. Costa a , M.M. Carnasciali b , R. Masini c a INFM-LAMIA and DCCI, Via Dodecaneso, 31, 16146 Genova, Italy b INSTM and DCCI, Via Dodecaneso, 31, 16146 Genova, Italy c CNR-IMEM, Via Dodecaneso, 33, 16146 Genova, Italy article info Article history: Received 27 July 2009 Received in revised form 7 January 2010 Accepted 9 January 2010 Available online 18 January 2010 Keywords: Mixed oxides Rare earth perovskites Thermodynamic stability abstract A stability study of perovskitic LaREO 3 oxides (RE = Dy, Ho, Er, Tm, Yb, Lu) as a function of temperature was undertaken. A correlation between the Goldschmidt t value and the perovskitic stability field amplitude was found: the latter widens as t increases. Magnetic measurements, performed on all the perovskitic samples, showed that t is also related to the exchange interactions. LaREO 3 oxides were synthesized by thermal decomposition of the corresponding coprecipitated oxalates at temperatures ranging between 600 and 1800 ◦ C. Simultaneous differential thermal and ther- mogravimetric analyses showed that all the La–RE mixed oxalates decompose similarly. All the oxides, except LaDyO 3 , crystallize in the perovskitic form in a temperature range that depends on the ionic size difference between La and the smaller rare earth; above and below the perovskitic stability field, the B or C form, typical of rare earth sesquioxides, is present. Rietveld refinements, carried out on all the LaREO 3 samples synthesized at 1200 ◦ C, showed the occurrence of an orthorhombic distorted perovskitic structure belonging to the Pnma space group. © 2010 Published by Elsevier B.V. 1. Introduction Interlanthanide perovskitic oxides constitute an important fam- ily of mixed oxides: in general RERE ′ O 3 (RE, RE ′ = rare earth elements) oxides, and in particular LaREO 3 , RE ′ doped LaREO 3 , are currently studied as protonic conductors [1,2], scintillators [3] and for their magnetic properties [4]. Besides, rare earths are com- monly used in solid state electrolytes [5] and since these oxides may undergo an insulating-conductor transition or can be potential hosts for luminescent ions, their electrical and optical properties could be tuned and studied by the introduction of another rare earth or a transition metal at the perovskitic B site [6,7]. The optical properties of rare earth oxides have been, in particular, thoroughly studied: it is for example well known that rare earth sesquioxides like Lu 2 O 3 , Gd 2 O 3 eY 2 O 3 , if properly doped, show interesting lumi- nescent properties [8,9,10]. The perovskitic structure, moreover, characteristic of ATiO 3 :Pr 3+ (A = Ca, Sr, Ba) [11] and GdAlO 3 :Dy 3+ [12], and more complex perovskitic cells, like SrRE 2 Al 2 O 7 :Eu 3+ , BaRE 2 Ti 3 O 10 :Eu 3+ and RETa 3 O 9 :Eu 3+ [13], are typical of many lumi- nescent materials. The luminescence intensity of an optically active ion is related to the crystal field symmetry of the host lattice and an increased crystal distortion leads to an enhanced emission inten- sity. The interlanthanide perovskites considered in this work are therefore ideal candidates as host lattices in the search for new ∗ Corresponding author. Tel.: +39 0103536101; fax: +39 0103628252. E-mail address: artini@chimica.unige.it (C. Artini). luminescent materials: they are in fact characterized by a strongly distorted crystal cell due to the small size difference between the two cations. Despite the usefulness of intra-rare earth perovskites, only few reports exist on their synthesis and characterization [4,14–17] and a stability assessment of these compounds is lacking. A stability study of the LaREO 3 perovskitic structure as a function of temper- ature was thus undertaken. Below about 2000 ◦ C RE 2 O 3 oxides can crystallize in three dis- tinct crystalline types: A (hexagonal), B (monoclinic) and C (cubic), depending on the RE cationic radius and temperature [18]. At 1200 ◦ C the hexagonal (A-type) structure is typical of the larger cations (from La to Nd), while the smaller (from Gd to Lu) and inter- mediate ions crystallize in cubic (C-type) and monoclinic structure (B-type) respectively. RE–RE ′ binary mixed oxides can crystallize in the same three structures, depending on the crystalline forms of starting RE 2 O 3 and RE ′ 2 O 3 , as well as on thermal treatment temper- ature. If RE 2 O 3 belongs to the C-type and RE ′ 2 O 3 to the A-type, three monophasic regions (A, B and C phases) and two biphasic regions (A + B and B + C) are present in the pseudobinary phase diagram, as their existence field depends on temperature [19]. If the differ- ence among ionic radii is large enough to satisfy the Goldschmidt tolerance factor t [20], a perovskitic compound forms close to the equimolar composition. The ideal perovskitic structure consists of a cubic unit cell (s. gr. Pm3m) where A is in twelvefold coordination and B in octahedral coordination with respect to the oxygen atoms. The ABO 3 general formula refers to a mixed oxide where A is a large cation and B 0925-8388/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.jallcom.2010.01.030