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Rare-earth perovskites along the CaTiO (sub 3) -Na (sub 0.5) La (sub 0.5) TiO (sub 3) join; phase transitions, formation enthalpies, and implications for loparite minerals

Dawei Feng, Radha Shivaramaiah and Alexandra Navrotsky
Rare-earth perovskites along the CaTiO (sub 3) -Na (sub 0.5) La (sub 0.5) TiO (sub 3) join; phase transitions, formation enthalpies, and implications for loparite minerals
American Mineralogist (September 2016) 101 (9): 2051-2056

Abstract

The mineral loparite is the principal ore of the light rare earth elements (LREE) with solid solution compositions in the quaternary system perovskite-lueshite-tausonite-loparite (CaTiO (sub 3) -NaNbO (sub 3) -SrTiO (sub 3) -Na (sub 0.5) Ln (sub 0.5) TiO (sub 3) ) (Ln = La, Ce, Pr, Nd). In this study, perovskite solid solutions Ca (sub 1-2x) Na (sub x) La (sub x) TiO (sub 3) (0 < or = x < or = 0.50) along the perovskite-loparite join were synthesized using solid-state sintering methods. XRD analysis indicates that as the Na+La content increases, the structure changes from orthorhombic to tetragonal. The enthalpies of formation at 298 K from the constituent oxides (Delta H (super o) (sub f,ox) ) and from the elements (Delta H (super o) (sub f,el) ) have been determined using high-temperature oxide melt solution calorimetry in molten 3Na (sub 2) O.4MoO (sub 3) at 973 K. Enthalpy of formation from oxides Delta H (super o) (sub f,ox) becomes more exothermic with increasing Na+La content, suggesting a stabilization effect of the substitution Ca (super 2+) --> 0.5Na (super +) + 0.5La (super 3+) . The observed trend of increasing thermodynamic stability with decreasing structural distortion is in agreement with that seen in many other ABO (sub 3) perovskites. The thermodynamic stability of perovskite solid solutions Ca (sub 1-2x) Na (sub x) La (sub x) TiO (sub 3) (0 < or = x < or = 0.50) along the CaTiO (sub 3) -Na (sub 0.5) La (sub 0.5) TiO (sub 3) join provides insights into the natural occurrence of loparite minerals (La,Na,Sr,Ca)(Ti,Nb,Ta,Fe (super 3+) )O (sub 3) .


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 101
Serial Issue: 9
Title: Rare-earth perovskites along the CaTiO (sub 3) -Na (sub 0.5) La (sub 0.5) TiO (sub 3) join; phase transitions, formation enthalpies, and implications for loparite minerals
Affiliation: University of California at Davis, Peter Rock Thermochemistry Laboratory, Davis, CA, United States
Pages: 2051-2056
Published: 201609
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 39
Accession Number: 2016-096420
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201647
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