The crystal chemistry of the M+VO3 (M+ = Li,Na, K,NH4, Tl,Rb, and Cs) pyroxenes

The crystal structures of M⁺VO₃(M⁺ = K, NH₄, and Cs) have been refined using three-dimensional counter-diffractometer X-ray data and full-matrix least-squares methods. The structure of these compounds is characterized by a (V⁵⁺O^2?₃)^?_∞ chain extending along the c-axis (Pbcm orientation), with adjacent chains linked by the alkali metal cation. The structure may be considered as a variant of the pyroxene structure, and standard atom nomenclature is proposed in order to facilitate comparison with silicate pyroxenes. Structural variation across this series is discussed in detail and is compared with the analogous M⁺M³⁺Si₂O₆ (M⁺ = Li, Na; M³⁺ = Al, Cr, Fe, Sc, In) series.     

Dielectric properties of some MM′O4 and MTiM′O6 (M=Cr, Fe, Ga; M′=Nb, Ta, Sb) rutile-type oxides

We describe an investigation of the structure and dielectric properties of MM′O₄ and MTiM′O₆ rutile-type oxides for M=Cr, Fe, Ga and M′=Nb, Ta and Sb. All the oxides adopt a disordered rutile structure (P4₂/mnm) at ambient temperature. A partial ordered trirutile-type structure is confirmed for FeTaO₄ from the low temperature (17 K) neutron diffraction studies. While both the MM′O₄ oxides (CrTaO₄ and FeTaO₄) investigated show a normal dielectric property MTiM′O₆ oxides for M=Fe, Cr and M′=Nb/Ta/Sb display a distinct relaxor/relaxor-like response. Significantly the corresponding gallium analogs, GaTiNbO₆ and GaTiTaO₆, do not show a relaxor response at T<500 K.     

Phase formation features in the systems M2MoO4-Fe2(MoO4)3 (M=Rb, Cs) and crystal structures of new double polymolybdates M3FeMo4O15

The systems M₂MoO₄-Fe₂(MoO₄)₃ (M=Rb, Cs) were shown to be non-quasibinary joins of the systems M₂O-Fe₂O₃-MoO₃. New compounds M₃FeMo₄O₁₅ were revealed along with the known MFe(MoO₄)₂ and M₅Fe(MoO₄)₄. The unit cell parameters of the new compounds are a=11.6192(2), b=13.6801(3), c=9.7773(2) Å, β=92.964(1)°, space group P2₁/c, Z=4 (M=Rb) and a=11.5500(9), b=9.9929(7), c=14.513(1) Å, β=90.676(2)°, space group P2₁/n, Z=4 (M=Cs). In the structures of M₃FeMo₄O₁₅ (M=Rb, Cs), a half of the FeO₆ octahedra share two opposite edges with two MoO₆ octahedra linked to other FeO₆ octahedra through the bridged MoO₄ tetrahedra by means of the common oxygen vertices to form the chains along the a axis. The difference between the structures is caused by diverse mutual arrangements of the adjacent polyhedral chains.     

Synthesis and characterization of phosphates in molten systems Cs2O-P2O5-CaO-M2O3 (M—Al, Fe, Cr)

The crystallization of complex phosphates from the melts of Cs₂O-P₂O₅-CaO-M^III₂O₃ (M^III—Al, Fe, Cr) systems have been investigated at fixed value Cs/P molar ratios equal to 0.7, 1.0 and 1.3 and Са/Р=0.2 and Ca/М^III=1. The fields of crystallization of CsCaP₃O₉, β-Ca₂P₂O₇, Cs₂CaP₂O₇, Cs₃CaFe(P₂O₇)₂, Ca₉M^III(PO₄)₇ (M^III—Fe, Cr), Cs_0.63Ca_9.63Fe_0.37(PO₄)₇ and CsCa₁₀(PO₄)₇ were determined. Obtained phosphates were investigated using powder X-ray diffraction and FTIR spectroscopy. Novel whitlockite-related phases CsCa₁₀(PO₄)₇ and Cs_0.63Ca_9.63Fe_0.37(PO₄)₇ have been characterized by single crystal X-ray diffraction: space group R3c, a=10.5536(5) and 10.5221(4) Å, с=37.2283(19) and 37.2405(17) Å, respectively.     

Synthesis and structure of In(IO3)3 and vibrational spectroscopy of M(IO3)3 (M=Al, Ga, In)

The reaction of Al, Ga, or In metals and H₅IO₆ in aqueous media at 180 °C leads to the formation of Al(IO₃)₃, Ga(IO₃)_3, or In(IO₃)₃, respectively. Single-crystal X-ray diffraction experiments have shown In(IO₃)₃ contains the Te₄O₉-type structure, while both Al(IO₃)₃ and Ga(IO₃)₃ are known to exhibit the polar Fe(IO₃)₃-type structure. Crystallographic data for In(IO₃)₃, trigonal, space group , a=9.7482(4) Å, c=14.1374(6) Å, V=1163.45(8) Z=6, R(F)=1.38% for 41 parameters with 644 reflections with I>2σ(I). All three iodate structures contain group 13 metal cations in a distorted octahedral coordination environment. M(IO₃)₃ (M=Al, Ga) contain a three-dimensional network formed by the bridging of Al³⁺ or Ga³⁺ cations by iodate anions. With In(IO₃)₃, iodate anions bridge In³⁺ cations in two-dimensional layers. Both materials contain distorted octahedral holes in their structures formed by terminal oxygen atoms from the iodate anions. The Raman spectra have been collected for these metal iodates; In(IO₃)₃ was found to display a distinctively different vibrational profile than Al(IO₃)₃ or Ga(IO₃)₃. Hence, the Raman profile can be used as a rapid diagnostic tool to discern between the different structural motifs.     

Reactivity of metal oxides: Thermal and photochemical dissolution of MO and MFe2O4 (M=Ni, Co, Zn)

Dissolution rates of NiO, CoO, ZnO, α-Fe₂O₃ and the corresponding ferrites in 0.1 mol dm⁻³ oxalic acid at pH 3.5 were measured at 70 °C. The dissolution of simple oxides proceeds through the formation of surface metal oxalate complexes, followed by the transfer of surface complexes (rate-determining step). At constant pH, oxalate concentration and temperature, the trend in the first-order rate constant for the transfer of the surface complexes (k_Me; Me=Ni, Co, Zn, Fe) parallels that of water exchange in the dissolved metal ions (k_−w). Thus, the most important factor determining the rates of dissolution of metal oxides is the lability of Me-O bonds, which is in turn defined by the electronic structure of the metal ion and its charge/radius ratio. UV (384 nm) irradiation does not increase significantly the dissolution rates of NiO, CoO and ZnO, whereas hematite is highly sensitive to UV light. For ferrites, the reactivity order is ZnFe₂O₄>CoFe₂O₄?NiFe₂O₄. Dissolution is congruent, with rates intermediate between those of the constituent oxides, Fe₂O₃ and MO (M=Co, Ni, Zn), reflecting the behavior of very thin leached layers with little Zn and Co, but appreciable amounts of Ni. The more robust Ni²⁺ labilizes less the corresponding ferrite. The correlation between log k_M and log k_−w is somewhat blurred and displaced to lower k_M values. Fe(II), either photogenerated or added as salt, enhances the rate of Fe(III) phase transfer. A simple reaction mechanism is used to interpret the data.     

The structural chemistry of Bi14MO24 (M=Cr, Mo, W) phases: bismuth oxides containing discrete MO4 tetrahedra

The phases Bi₁₄MO₂₄ (M=Cr, Mo, W) have been studied using differential scanning calorimetry, variable temperature X-ray powder diffraction and neutron powder diffraction. All three compounds were found to undergo a phase change, on cooling, from the previously reported tetragonal symmetry (I4/m) to monoclinic symmetry (C2/m). Transition temperatures were determined to be ∼306 K (M=W) and ∼295 K (M=Mo), whereas a gradual transition between 275 and 200 K was observed for M=Cr. The high and low temperature structures are very similar, as indicated by the relationship between the monoclinic and tetragonal unit cell parameters: a_m=√2a_t, b_m=c_t, c_m=a_t, β∼135°. High-resolution neutron powder diffraction data, collected at 400 and 4 K, were used to establish the nature of the transition, which was found to involve a reduction in the statistical possibilities for orientation of the MO₄ tetrahedra. However, in both tetragonal and monoclinic variants, a degree of orientational disorder of the tetrahedra occurs to give partially occupied sites in the average unit cell.     

Structure of two new borates YCa3(AlO)3(BO3)4 and YCa3(GaO)3(BO3)4

By replacing Mn in YCa₃(MnO)₃(BO₃)₄ with trivalent Al and Ga, two new borates with the compositions of YCa₃(MO)₃(BO₃)₄ (M=Al, Ga) were prepared by solid-state reaction. Structure refinements from X-ray powder diffraction data revealed that both of them are isostructural to gaudefroyite with a hexagonal space group P6₃/m. Cell parameters of a=10.38775(13)Å, c=5.69198(10)Å for the Al-containing compound and a=10.5167(3)Å, c=5.8146(2)Å for the Ga analog were obtained from the refinements. The structure is constituted of AlO₆ or GaO₆ octahedral chains interconnected by BO₃ groups in the ab plane to form a Kagomé-type lattice, leaving trigonal and apatite-like tunnels. It is found that most rare-earth and Cr, Mn ions can be substituted into the Y³⁺ and M³⁺ sites, respectively, and the preference of rare-earth ions to locate in the trigonal tunnel is correlated to the sizes of the M³⁺ ions.     

Phase relationships in the systems MSCr2S3In2S3 (M = Co,Cd, Hg)

The phase diagrams of the quaternary systems MSCr₂S₃In₂S₃, with M = Co, Cd, and Hg, were studied with the help of X-ray powder photographs of quenched samples, high-temperature X-ray diffraction patterns, DTA and TG measurements, and far-infrared spectra. Because indium sulfides do react with silica tubes, alumina crucibles must be used for annealing the samples. Complete series of mixed crystals are formed among the spinel-type compounds MCr₂S₄, MIn₂S₄ (M = Cd, Hg), and In₂S₃. HgIn₂S₄ is decomposed at temperatures above 300°C. In the sections CoCr₂S₄CoIn₂S₄ and CoCr₂S₄In₂S₃ relatively large miscibility gaps exist due to the change from normal to inverse spinel structure. But the interchangeability of both systems increases with increasing temperature, and at temperatures above 1000°C, complete series of solid solutions are formed, which can be quenched to ambient temperature. Superstructure ordering like that of ordered α-In₂S₃ has been found in the In-rich region of the MIn₂S₄In₂S₃ solid solutions. The unit cell dimensions of all stoichiometric and phase boundary compounds, e.g., Cd_1.15In_1.9S₄, including the chromium spinels MCr₂S₄ (M = Mn, Zn) and ZnCr₂Se₄, are given and discussed in terms of possible deviations from stoichiometry.     

Synthesis, structure and physical properties of Ru ferrites: BaMRu5O11 (M=Li and Cu) and BaM′2Ru4O11 (M′=Mn, Fe and Co)

The synthesis, structure, and physical properties of five R-type Ru ferrites with chemical formula BaMRu₅O₁₁ (M=Li and Cu) and BaM′₂Ru₄O₁₁ (M′=Mn, Fe and Co) are reported. All the ferrites crystallize in space group P6₃/mmc and consist of layers of edge sharing octahedra interconnected by pairs of face sharing octahedra and isolated trigonal bipyramids. For M=Li and Cu, the ferrites are paramagnetic metals with the M atoms found on the trigonal bipyramid sites exclusively. For M′=Mn, Fe and Co, the ferrites are soft ferromagnetic metals. For M′=Mn, the Mn atoms are mixed randomly with Ru atoms on different sites. The magnetic structure for BaMn₂Ru₄O₁₁ is reported.     

Etude structurale des composés MxTiSe2 (M = Fe,Co, Ni)

New compounds M_xTiSe₂ have been prepared with M = Fe (x ? 0.66), M = Co or Ni (x ? 0.50). The metal M is located in vacant octahedral sites of the TiSe₂ host lattice (hexagonal unit cell a′, c′). An ordering of vacancies occurs if x ? 0.20. With M = Co or Ni (x = 0.50) and with M = Fe (0.25 ? x ? 0.66) isotypic compounds of Ti₃Se₄ can be obtained (M₃ □ X₄ type; monoclinic unit cell a ≈ a′ √3, b ≈ a′, c ≈ 2c′). The compounds Fe_0.38TiSe₂ and Co_0.38TiSe₂ (hexagonal unit cell a ≈ a′ √3, c ≈ 2c′) are of the M₂ □ X₃ type, variety 2c′. The Fe_0.25TiSe₂ and Co_0.25TiSe₂ monoclinic unit cells (a ≈ 2a′ √3, b ≈ 2a′, c ≈ 2c′) allow us to assume, for these two compounds, a structure of the M₅ □ ₃X₈ type, variety 2c′, identical to the Ti₅Se₈ one. The compound Ni_0.25TiSe₂ has an hexagonal unit cell (a ≈ 2a′, c ≈ 3c′); it belongs to a so-called 3c′ variety of the M₅ □ ₃X₈ type.     

Magnetoresistance Study on TPP[M(Pc)(CN)2]2 (M=Fe, Co, Fe0.30Co0.70) Salts

The magnetoresistance study on TPP[M(Pc)(CN)₂]₂ (M=Fe, Co, Fe_0.30Co_0.70) salts is reported. These three salts have similar columnar structures, nevertheless exhibit different electrical behaviors. TPP[Fe(Pc)(CN)₂]₂ exhibits anisotropic giant negative magnetoresistance, while TPP[Co(Pc)(CN)₂] exhibits large positive magnetoresistance. The alloyed compound, TPP[Fe_0.30Co_0.70 (Pc)(CN)₂]₂, also exhibits anisotropic negative magnetoresistance, although the decrease in the resistivity under the magnetic field is less than that of TPP[Fe(Pc)(CN)₂]₂. The g-tensor anisotropy in the [Fe(Pc)(CN)₂] unit qualitatively explains the field-orientation dependence of the negative magnetoresistance. Magnetic fluctuation associated with a weak-ferromagnetic transition is suggested as a possible origin of the giant negative magnetoresistance.     

Crystal structures of lazulite-type oxidephosphates TiTi3O3(PO4)3 and M4Ti27O24(PO4)24 (M=Ti, Cr, Fe)

Single crystals of the oxidephosphates Ti^IIITi^IV₃O₃(PO₄)₃ (black), Cr^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (red-brown, transparent), and Fe^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (brown) with edge-lengths up to 0.3 mm were grown by chemical vapour transport. The crystal structures of these orthorhombic members (space group F2dd ) of the lazulite/lipscombite structure family were refined from single-crystal data [Ti^IIITi^IV₃O₃(PO₄)₃: Z=24, a=7.3261(9) Å, b=22.166(5) Å, c=39.239(8) Å, R₁=0.029, wR₂=0.084, 6055 independent reflections, 301 variables; Cr^III₄Ti^IV₂₇O₂₄(PO₄)₂₄: Z=1, a=7.419(3) Å, b=21.640(5) Å, c=13.057(4) Å, R₁=0.037, wR₂=0.097, 1524 independent reflections, 111 variables; Fe^III₄Ti^IV₂₇O₂₄(PO₄)₂₄: Z=1, a=7.4001(9) Å, b=21.7503(2) Å, c=12.775(3) Å, R₁=0.049, wR₂=0.140, 1240 independent reflections, 112 variables). For Ti^IIITi^IVO₃(PO₄)₃ a well-ordered structure built from dimers [Ti^III,IV₂O₉] and [Ti^IV,IV₂O₉] and phosphate tetrahedra is found. The metal sites in the crystal structures of Cr₄Ti₂₇O₂₄(PO₄)₂₄ and Fe₄Ti₂₇O₂₄(PO₄)₂₄, consisting of dimers [M^IIITi^IVO₉] and [Ti^IV,IV₂O₉], monomeric [Ti^IVO₆] octahedra, and phosphate tetrahedra, are heavily disordered. Site disorder, leading to partial occupancy of all octahedral voids of the parent lipscombite/lazulite structure, as well as splitting of the metal positions is observed. According to Guinier photographs Ti^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (a=7.418(2) Å, b=21.933(6) Å, c=12.948(7) Å) is isotypic to the oxidephosphates M^III₄Ti^IV₂₇O₂₄(PO₄)₂₄ (M^III: Cr, Fe). The UV/vis spectrum of Cr₄Ti₂₇O₂₄(PO₄)₂₄ reveals a rather small ligand-field splitting Δ_o=14,370 cm⁻¹ and a very low nephelauxetic ratio β=0.72 for the chromophores [Cr^IIIO₆] within the dimers [Cr^IIITi^IVO₉].     

Neue Verbindungen mit Granatstruktur. IV. Arsenate des Typs {Na3}[M2III](As3)O12

The preparation and thermal behaviour of two garnets of the type {Na₃}[M^III₂]·(As₃)O₁₂ (M^III = Cr, Fe) are described. Both compounds undergo a reversible conversion into a high-temperature phase which, for the case of M^III = Ga, is found as the sole structure type. There is no mixed-crystal formation between {Y₃}[Fe₂](Fe₃)O₁₂ and {Na₃}[Fe₂] · (As₃)O₁₂ Preliminary investigations were performed on the possible mixed-crystal formation between the Ga compound and the Cr and Fe garnet structure, respectively.     

Hexagonal-spinel transitions in antimonates Li2Cr3−xMIIIxSbO8 (MIII = Al,Fe, Ga)

A new family of antimonates Li₂Cr_3?xM^III_xSbO₈ (M^III = Al, Fe, Ga) was synthesized and studied by X-ray diffraction and ir spectroscopy. The Al-containing compounds exhibit a hexagonal close-packed structure similar to that of LiFeSnO₄ ($\text{a ? 5.8, c ? 9.5}$ Å. For M = Fe or Ga, two structural forms are isolated: a low-temperature hexagonal form which is isotypic with LiFeSnO₄ and a high-temperature cubic form isotypic with the spinel structure. The hexagonal spinel transformation was observed for the first time, while the reverse transformation cannot be obtained.     

Crystal chemistry of MSeO3 and MTeO3 (M = Mg,Mn, Co,Ni, Cu,and Zn)

New selenites and tellurites MgSeO₃, MnSeO₃, CoSeO₃, NiSeO₃, CuSeO₃, MnTeO₃, CoTeO₃, and NiTeO₃ were synthesized under high pressures and temperatures. All the compounds are isomorphous and their crystal system is orthorhombic. Structure analyses were carried out for all the selenites and CoTeO₃. The structure is described as a salt of M²⁺ and SeO₃^2? or TeO₃^2? ions or as a distorted perovskite. In these compounds an Se or Te atom is closely linked to three oxygen atoms to form a flattened trigonal pyramid. The features of this coordination are discussed. At low temperature, magnetic order appears in all the compounds containing iron group ions, among which CuSeO₃ is a ferromagnet with the Curie temperature of 26°K.     

Ternary silicides M2Cr4Si5 (M=Ti, Zr, Hf): filled variants of the Ta4SiTe4 structure type

The isostructural ternary silicides M₂Cr₄Si₅ (M=Ti, Zr, Hf) were prepared by arc-melting of the elemental components. The single-crystal structure of Zr₂Cr₄Si₅ was determined by X-ray diffraction (Pearson symbol oI44, orthorhombic, space group Ibam, Z=4, a=7.6354(12) Å, b=16.125(3) Å, c=5.0008(8) Å). Zr₂Cr₄Si₅ adopts the Nb₂Cr₄Si₅-type structure, an ordered variant of the V₆Si₅-type structure. It consists of square antiprisms that have Zr and Cr atoms at the corners and Si atoms at the centers; they share opposite faces to form one-dimensional chains _∞¹[Zr_4/2Cr_4/2Si] surrounded by additional Si atoms and extending along the c direction. In a new interpretation of the structure, additional Cr atoms occupy interstitial octahedral sites between these chains, clarifying the relation between this structure and that of Ta₄SiTe₄. The formation of short Si-Si bonds in Zr₂Cr₄Si₅ is contrasted with the absence of Te-Te bonds in Ta₄SiTe₄. The compounds M₂Cr₄Si₅ (M=Ti, Zr, Hf) exhibit metallic behavior and essentially temperature-independent paramagnetism. Bonding interactions were analyzed by band structure calculations, which confirm the importance of Si-Si bonding in these metal-rich compounds.     

Ferrimagnetism in the rare earth titanium (III) oxides, RTiO3; R = Gd,Tb, Dy,Ho, Er,Tm

The series of compounds RTiO₃, R = Gd, Tb, Dy, Ho, Er, and Tm, were obtained as single-phase materials via solid state reaction between Ti₂O₃ and R₂O₃ at ca. 1500°C in welded molybdenum crucibles under argon; YbTiO₃ and LuTiO₃ could not be obtained as single-phase materials using this procedure. Lattice constants for all compounds were determined from powder X-ray data and are compared with previous results. All of these materials order magnetically between 30 and 70 K. From the appearance of the χ^?1_m vs T curve the type of order can be identified as ferrimagnetic. High-temperature susceptibility data have been fit to a two-sublattice molecular field model and the intra- and intersublattice interaction constants have been extracted. It is found that the TiTi interaction is ferromagnetic and relatively constant from R = Gd to R = Lu. Low-temperature magnetization field data suggest the existence of complex magnetic structures, large magnetocrystalline anisotropy, or both. The magnetic properties of the RTiO₃ series are compared to those of the chemically similar and better-known RMO₃ phases where M = Al, V, Cr, Mn, and Fe. The observed differences are shown to follow from the sign of the M-M interaction, which is ferromagnetic for M = Ti and antiferromagnetic for M = V, Cr, Mn, and Fe, together with the implications of the crystal symmetry for the R-M interaction.     

Phase formation in the systems Ag2MoO4-MO-MoO3 (M=Ca, Sr, Ba, Pb, Cd, Ni, Co, Mn) and crystal structures of Ag2M2(MoO4)3 (M=Co, Mn)

Phase equilibria in the systems Ag₂MoO₄-MMoO₄ (M=Ca, Sr, Ba, Pb, Ni, Co, Mn) and subsolidus phase relations in the systems Ag₂MoO₄-MO-MoO₃ (M=Ca, Pb, Cd, Mn, Co, Ni) were investigated using XRD and thermal analysis. The systems Ag₂MoO₄-MMoO₄ (M=Ca, Sr, Ba, Pb, Ni) belong to the simple eutectic type whereas in the systems Ag₂MoO₄-MMoO₄ (M=Co, Mn) incongruently melting Ag₂M₂(MoO₄)₃ (M=Co, Mn) were formed. In the ternary oxide systems studied no other compounds were found. Low-temperature LT-Ag₂Mn₂(MoO₄)₃ reversibly converts into the high-temperature form of a similar structure at 450-500°C. The single crystals of Ag₂Co₂(MoO₄)₃ and LT-Ag₂Mn₂(MoO₄)₃ were grown and their structures determined (space group , Z=2; lattice parameters are a=6.989(1) Å, b=8.738(2) Å, c=10.295(2) Å, α=107.67(2)°, β=105.28(2)°, γ=103.87(2)° and a=7.093(1) Å, b=8.878(2) Å, c=10.415(2) Å, α=106.86(2)°, β=105.84(2)°, γ=103.77(2)°, respectively) and refined to R(F)=0.0313 and 0.0368, respectively. The both compounds are isotypical to Ag₂Zn₂(MoO₄)₃ and contain mixed frameworks of MoO₄ tetrahedra and pairs of M²⁺O₆ octahedra sharing common edges. The Ag⁺ ions are disordered and located in the voids forming infinite channels running along the a direction. The peculiarities of the silver disorder in the structures of Ag₂M₂(MoO₄)₃ (M=Zn, Mg, Co, Mn) are discussed as well as their relations with analogous sodium-containing compounds of the structural family of Na₂Mg₅(MoO₄)₆. The phase transitions in Ag₂M₂(MoO₄)₃ (M=Mg, Mn) of distortive or order-disorder type are suggested to have superionic character.     

The production of Cr(NH3)6 from the acid catalysed hydrolysis of Cr(NH3)5(NCO)

The rate of the reaction

has been investigated at 40–65°C with [HClO₄] varying from 0.04 to 0.6 M (μ = 0.6 M, NaClO₄). The observed rate law has the form: -d[Cr(NH₃)₅(NCO)²⁺]/dt = k_obs[Cr(NH₃)₅(NCO)²⁺] where k_obs = a[H+]²{1 + b[H⁺]²} and ^?1 at 55.0°C, a = 0.36 M^?1 s^?2 and b = 6.9 × 10^?3 M^?1 s^?1. The rate of loss of Cr(NH₃)₅(NCO)²⁺ increases with increasing acidity to a limiting value (at [H⁺] ～ 0.5 M) but the yield of Cr(NH₃)₆³⁺ decreases with increasing [H⁺] and increases with increasing temperature. In the kinetic studies the maximum yield of Cr(NH₃)₆³⁺ was 35% but a synthetic procedure has been developed to give a 60% yield.