Mild Hydrothermal Syntheses and Thermal Behaviors of Hydrogarnets Sr₃M₂(OH)₁₂ (M = Cr, Fe, and Al)

Abstract

Hydrogarnets Sr3M2(OH)12 (M = Cr, Fe, and Al) were synthesized from mild hydrothermal systems at 240 °C with Sr(OH)2·8H2O, Cr(NO3)3·9H2O, Fe(NO3)3·9H2O, and Al(OH)3 as the starting materials. High solubility of the input species and basic concentration in the reaction systems considerably lower crystallization temperatures and enhance the crystallinity of the resulting hydrogarnets. The structural stability and thermal properties of Sr based hydrogarnets were investigated by X-ray diffraction, DTA-TG, infrared, and X-ray photoelectron spectroscopies. With increasing temperature, Sr based hydrogarnets underwent fast dehydration due to the loss of the nearest OH- species of Sr−OH dodecahedra. During the course of decomposition, valence variations of the hydrogarnet framework ions, e.g., Fe and Cr directly determined the decomposition process and phase compositions in final decomposition products. The decomposition product of Sr3Cr2(OH)12 in air was a mixture of SrCrO4 and SrO, but in N2, a mixture of a rhombohedral phase Sr4Cr2O8 with some amorphous phase was observed; in either O2 or N2, Sr3Fe2(OH)12 decomposed to an unknown phase, which was not a perovskite Sr3Fe2O7 - x . Sr3Al2(OH)12 decomposed to a single phase Sr3Al2O6. In Sr−Fe hydrogarnet, two different symmetric octahedra of Fe(OH)6 were revealed by Mossbauer spectroscopy. Mild hydrothermal crystallization and the thermal decomposition behaviors of Sr-based hydrogarnets are discussed.

Syntheses, Crystal Structures and Physical Properties of Chromium and Rhodium Hydrogarnets Ca3[Cr(OH)6]2, Sr3[Cr(OH)6]2 and Sr3[Rh(OH)6]2

“…The IR spectra of the three hydrogarnets (Figure ) show sharp bands, characteristic for the O–H valence vibrational modes, at 3625 cm –1 for Sr 3 [Cr(OH) 6 ] 2 , 3616 cm –1 for Ca 3 [Cr(OH) 6 ] 2 and 3592 cm –1 for Sr 3 [Rh(OH) 6 ] 2 . The signals are sharp without apparent peak split or multiplet character, which confirms the evidence of O–H groups.…”

“…The short‐range mass increase after the peak of the water loss in the DTA reflects this oxygen uptake. Li et al already stated, that atmospheric oxygen is needed for the oxidation of chromium(III) in the formation of strontium chromate(VI) by thermal analyses …”

“…For example, the decomposition of strontium katoite Sr 3 [Al(OH) 6 ] 2 to Sr 3 Al 2 O 6 takes place at about 600 °C, whereas the solid‐state reaction of Al 2 O 3 with SrCO 3 to Sr 3 Al 2 O 6 necessitates at least 1200 °C and several intermediate grinding steps , . Similarly, the hydrogarnet Sr 3 [Fe(OH) 6 ] 2 decomposes into SrFeO 3– δ below 655 °C, whereas above this temperature Sr 4 Fe 3 O 10– δ is formed .…”

New Materials in Hydrothermal Synthesis

“…Table 1 summarizes conditions for selected complex oxides and fluorides by hydrothermal synthesis in our laboratory. [35][36][37][38][39][40][41][42][43] Perovskites, ABO 3 (A ) Ca, Sr, Ba, Pb; B ) Ti, Sn, Zr), were hydrothermally prepared; 35 complex oxides containing rare earth ions, e.g., Na x La 2/3-x/3 TiO 3 , Na x Ag y La 2/3-(x+y)/3 -TiO 3 and Na x Li y La 2/3-(x+y)/3 TiO 3 , synthesized hydrothermally at 200-240°C show enhanced electrical conductivity. 36 Further, giant magnetoresistance (GMR) materials, La 1-x A x MnO 3 (A ) Ca, Sr, and Ba), were recently synthesized from hydrothermal systems at 240°C; for example, the Ba analogue, La 0.5 Ba 0.5 MnO 3 , was obtained by heating an aqueous reaction mixture containing KMnO 4 and MnSO 4 reactants.…”

Hydrothermal Synthesis, Crystal Structure Determination, and Magnetic Properties of a New Calcium Iron Iridium Hydrogarnet Ca3(Ir2–xFex)(FeO4)2–x(O4H4)1+x with 0 ≤ x ≤ 1

“…Hydroxide groups as structural building units are well‐known from hydrogarnet minerals and are generally referred to as hydrogarnet defects: tetrahedral groups of four hydroxide anions – commonly denoted as (O 4 H 4 ) 4– – replace the iso‐charged SiO 4 4– tetrahedra. Hydrogrossulars, e.g., can be formulated as solid solution series Ca 3 Al 2 (SiO 4 ) 3– x (O 4 H 4 ) x and are known to form under hydrothermal conditions , . The end‐member of this series is the silicon‐free mineral katoite, Ca 3 Al 2 □ 3 (O 4 H 4 ) 3 where all tetrahedral positions are vacant (□).…”