Reduction of U(VI) in POCl<Subscript>3</Subscript>-lewis acid solutions

Features of the behavior of uranyl ions in POC1₃-MCl _x -²³⁵UO ₂₊ ² solutions (M = Ti, Si, Zr, Sn, Sb) were considered. Irreversible accumulation of U(IV) in the course of synthesis of POCl₃-SnCl₄-²³⁵UO ₂₊ ² solutions prepared from water-containing U(VI) compounds, excluding UO₂(ClO₄)₂ · 5H₂O, was found. The reaction rate increases with increasing uranyl concentration, k _l[U(IV)] ～ (1.6±0.2) × 10^?6 s^?1 (T = 380 K). The U(IV) accumulation was also observed on heating (T = 360–380 K) POCl₃-SnCl₄-²³⁵UO ₂₊ ² and POCl₃-SbCl₅-²³⁵UO ₂₊ ² solutions hermetically sealed in glass cells and on irradiating them by the light of a xenon lamp. In POCl₃-SbCl₅-²³⁵UO ₂₊ ² solutions prepared from UO₂(C1O₄)₂ · 5H₂O, U(IV) disappears within several days after stopping the irradiation. The reduction of U(VI) is caused by formation of uranyl dichlorophosphate complexes and by deactivation of uranyl excitation with chlorine-containing agents.     

Synthesis and properties of laser-active liquids POCl<Subscript>3</Subscript>-SbCl<Subscript>5</Subscript>-<Superscript>235</Superscript>UO<Stack><Subscript>2</Subscript><Superscript>2+</Superscript></Stack>-Nd<Superscript>3+</Superscript>

The effect of the synthesis conditions on the properties of inorganic laser-active liquids POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ is considered. The kinetic dependences of the U(IV) content and decay time of the Nd³⁺ luminescence in POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions for various synthesis procedures at 380 K have been obtained. In POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions, nonradiative energy transfer Nd³⁺ → U⁴⁺ is observed, and quenching of the Nd³⁺ luminescence is described by the Stern-Volmer law: k _q = (6.4 ± 0.6) × 10⁵ l mol^?1 s^?1. Laser liquids POCl₃-SbCl₅-²³⁵UO ₂ ²⁺ -Nd³⁺ with neodymium concentration of up to 0.7 M, uranyl concentration of up to 0.1 M, and decay time of the Nd³⁺ luminescence of up to 220 μs have been prepared for the first time.     

Synthesis and properties of liquid luminophores based on the POCl<Subscript>3</Subscript>-BiCl<Subscript>3</Subscript>-MCl<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> system

Stable BiCl₃-containing solutions of phosphorus oxychloride, activated with UO ₂ ²⁺ and Nd³⁺ ions, can be prepared only in the presence of another Lewis acid MCl _x . The electronic absorption spectra of the liquids prepared and the decay times of the Nd³⁺ luminescence are characteristic of individual solutions based on POCl₃-MCl _x . The radiation-chemical yield of Nd³⁺ in the excited state ⁴ F _3/2 in POCl₃-BiCl₃-MCl _x -²³⁵UO ₂ ²⁺ -Nd³⁺ solutions upon homogeneous excitation with uranium α-particles is lower than in POCl₃-MCl _x -²³⁵UO ₂ ²⁺ -Nd³⁺ solutions at comparable component concentrations. Apparently, Bi³⁺ in solutions based on the POCl₃-BiCl₃-MCl _x system is not incorporated in neodymium- and/or uranyl-containing complexes and remains in the matrix.     

Synthesis and crystal structure of [UO<Subscript>2</Subscript>(OH)(CO(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>)<Subscript>3</Subscript>]<Subscript>2</Subscript>(ClO<Subscript>4</Subscript>)<Subscript>2</Subscript>

The complex [UO₂(OH)(CO(NH₂)₂)₃]₂(ClO₄)₂ (I) was synthesized. A single crystal X-ray diffraction study showed that compound I crystallizes in the triclinic system with the unit cell parameters a = 7.1410(2), b = 10.1097(2), c = 11.0240(4) Å, α = 104.648(1)°, β = 103.088(1)°, γ = 108.549(1)°, space group \(P\bar 1\), Z = 1, R = 0.0193. The uranium-containing structural units of the crystals are binuclear groups [UO₂(OH)· (CO(NH₂)₂)₃] ₂ ²⁺ belonging to crystal-chemical group AM²M ₃ ¹ [A = UO ₂ ²⁺ , M² = OH^?, M¹ = CO(NH₂)₂] of uranyl complexes. The crystal-chemical analysis of nonvalent interactions using the method of molecular Voronoi-Dirichlet polyhedra was performed, and the IR spectra of crystals of I were analyzed.     

Reduction of U(VI) in laser liquids POCl3-SnCl4-235UO 2 2+ -Nd3+

U(IV) is irreversibly accumulated during synthesis of laser liquids POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ prepared from various initial Nd(III) and U(VI) compounds, irrespective of the way of their introduction. The rate of U(IV) accumulation in POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions increases with increasing UO ₂ ²⁺ and Nd³⁺ concentrations; for laser liquids with the Nd³⁺ luminescence lifetime τ > 150 μs the observed rate constant of U(IV) accumulation by the second-order reaction k ₂[U⁴⁺] is equal to (3 ± 1) × 10^?5 1 mol^?1 s^?1 at T = 380 K. U(IV) is accumulated during storage of POCl₃-SnCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions in hermetically sealed glass cells at room temperature and upon irradiation of solutions by xenon lamp light in the spectral region of UO ₂ ²⁺ absorption. The U(VI) reduction proceeds by chemical and photochemical activation of uranyl with formation of stable U⁴⁺ complexes with dichlorophosphate ions and also with Nd³⁺. Deactivation of the uranyl ion excitation with proton-and chlorine-containing impurities prevents U(VI) reduction.     

Synthesis and structure of (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>[UO<Subscript>2</Subscript>(CH<Subscript>3</Subscript>COO)<Subscript>3</Subscript>]<Subscript>2</Subscript>(NCS)

The compound (NH₄)₃[UO₂(CH₃COO)₃]₂(NCS) (I) was synthesized and examined by single crystal X-ray diffraction analysis. The compound crystallizes in the rhombic system with the unit cell parameters a = 11.5546(4), b = 18.5548(7), c = 6.7222(3) Å, V = 1441.19(10) ų, space group P2₁2₁2, Z = 2, R = 0.0345. The uranium-containing structural units of crystals of I are isolated mononuclear groups [UO₂(CH₃COO)₃]^? belonging to crystal-chemical group AB ₃ ⁰¹ (A = UO ₂ ²⁺ , B⁰¹ = CH₃COO^?) of uranyl complexes. The specific features of packing of the uranium-containing complexes in the crystal structure are considered.     

Interaction of aqueous UO<Stack><Subscript>2</Subscript><Superscript>2+</Superscript></Stack> solutions with sorbents based on modified silica gel containing Cu,Ni, and Zn

Interaction of aqueous UO ₂ ²⁺ solutions with modified sorbents based on coarsely porous silica gel MSKG, containing Cu, Ni, and Zn ions, was studied. Uranyl ions are sorbed on all the sorbents. The decontamination factors of 10^?2 M aqueous UO ₂ ²⁺ solutions on straight MSKG and on MSKG modified with Cu, Ni, and Zn are of the same order of magnitude and do not exceed 100. In the case of 1.1 × 10^?1 M UO ₂ ²⁺ solutions, the decontamination factors on straight MSKG and on MSKG modified with Cu, Ni, and Zn are also of the same order of magnitude but do not exceed 10. Interaction of the modified Ni-containing sorbent with a UO ₂ ²⁺ solution results in formation of a swamp-green precipitate of the composition NiU(OH)₆·4N₂H₅OH, i.e., UO ₂ ²⁺ is reduced to U⁴⁺.     

Crystal structure of KNa<Subscript>3</Subscript>[(UO<Subscript>2</Subscript>)<Subscript>5</Subscript>O<Subscript>6</Subscript>(SO<Subscript>4</Subscript>)]

The crystal structure of a previously unknown compound KNa₃[(UO₂)₅O₆(SO₄)] [space group Pbca, a = 13.2855(15), b = 13.7258(18), c = 19.712(2) Å, V = 3594.6(7) ų] was solved by direct methods and refined to R ₁ = 0.055 for 3022 reflections with |F _hkl | ≥ 4σ |F _hkl |. In the structure there are five sym-metrically nonequivalent uranyl cations. They are linked by cationcation (CC) interactions to form a pentamer whose central cation is U(2)O ₂ ²⁺ forming two three-centered CC bonds. All the uranyl ions are coordinated in the equatorial plane by five O atoms, which leads to the formation of pentagonal bipyramids sharing common edges to form layers parallel to the (100) plane. The sulfate tetrahedron links the uranyl layers into a 3D framework. The K⁺ and Na⁺ cations are arranged in framework voids. A brief review of CC interactions in U(VI) compounds is presented.     

Liquid luminophores POCl3-SiCl4-235UO 2 2+ -Nd3+

Samples of liquid luminophores POCl₃-SiCl₄-Nd³⁺, POCl₃-SiCl₄-²³⁵UO ₂ ²⁺ , and POCl₃-SiCl₄-²³⁵UO ₂ ²⁺ Nd³⁺ were prepared for the first time. The physicochemical properties and absorption spectra of the samples were compared. The solubility of Nd and U(VI) compounds in the POCl₃ SiCl₄ binary solvent was examined. The Nd³⁺ concentration attains a maximum of 0.3 mM when Nd and U(VI) trifluoroacetates are jointly dissolved at the [POCl₃]/[SiCl₄] molar ratio of 6 8. The shapes of the absorption band of the Nd ions at 860–900 nm, corresponding to the ⁴ I _9/2 → ⁴ F _3/2 electronic transition, differ significantly in POCl₃-SiCl₄-Nd³⁺ and POCl₃-SnCl₄-Nd³⁺ solutions, and in POCl₃-SiCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions the band shape also depends on the type of the Nd and U(VI) compounds introduced. The near-IR region of the absorption spectra of POCl₃-SiCl₄-²³⁵UO ₂ ²⁺ and POCl₃-SiCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions contains absorption bands of the OH groups. The band intensity is analyzed in relation to the characteristics of the solutions. The Nd³⁺ luminescence lifetime τ in the luminophores synthesized was estimated at 70–20 μs.     

Synthesis,phase formation study and magnetic properties of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowder prepared by mechanical milling

In this work we report the phase formation and magnetic properties of CoFe₂O₄ nanopowder prepared by mechanical alloying technique using metallic cobalt and hematite powder (1:1 molar ratio) as the initial raw material in ambient air atmosphere. The formation of single phase cobalt ferrite of (Co _0.18 ²⁺ Fe _0.82 ³⁺ )[Co _0.82 ²⁺ Fe _1.18 ³⁺ ]O₄ stoichiometry was confirmed for the samples milled above 15 h without any heat-treatment by XRD and Mössbauer techniques. The average crystallite size of the sample milled for 30 h was ～13 nm. The highest room temperature value of the magnetization measured at 1.5 T was 51 e.m.u/g for the sample milled for 25 h which was much lower than the corresponding value of the bulk cobalt ferrite (80.8 e.m.u/g at 300 K) due to the size effect.     

Crystal structure of [CH<Subscript>3</Subscript>NH<Subscript>3</Subscript>][(UO<Subscript>2</Subscript>)(H<Subscript>2</Subscript>AsO<Subscript>4</Subscript>)<Subscript>3</Subscript>]

The crystal structure of a previously unknown compound [CH₃NH₃][(UO₂)(H₂AsO₄)₃] was solved by direct methods and refined to R ₁ = 0.038 for 3041 reflections with |F _hkl | >-4σ |F _hkl |. The compound crystallizes in the monoclinic system, space group P2₁/c, a = 8.980(1), b = 21.767(2), c = 7.867(1) Å, β = 115.919(5)°, V = 1383.1(3) ų, Z = 4. In the structure of the compound, pentagonal bipyramids of uranyl ions, sharing bridging atoms with tetrahedral [H₂AsO₄]^? anions, form strongly corrugated layered complexes [(UO₂)(H₂AsO₄)₃]^? arranged parallel to the (100) plane. The protonated methylamine molecules [CH₃NH₃]⁺ form unidimensional tapelike packings parallel to the c axis and linked by hydrophilic-hydro-phobic interactions. The topology of the layered uranyl arsenate complex [(UO₂)(H₂AsO₄)₃]^? is unusual for uranyl compounds and was not observed previously. A specific feature of this topology is the presence of monodentate arsenate “branches” arranged within the layer.     

Formation-decomposition equilibrium of the NpO<Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack>·UO<Stack><Subscript>2</Subscript><Superscript>2+</Superscript></Stack> complex in chloride melts

The absorption spectra of the NpO ₂ ⁺ (5f ²) ion were examined in the region of the 3H ₅ ← ³ H ₄ magnetic dipole transition (1530–1760 nm) for series of melts with the UO ₂ ²⁺ concentration varied in the opposite directions: (1) NaCl-2CsCl eutectic melt with growing additions of the Cs₂UO₂Cl₄ complex salt and (2) Cs₂UO₂Cl₄ melt with growing additions of the NaCl-2CsCl mixture. Measurements of the integrated intensities of the bands belonging to the NpO ₂ ⁺ ·UO ₂ ²⁺ complex and unbound NpO ₂ ⁺ throughout the UO ₂ ²⁺ concentration range examined (up to 4.4 M in neat Cs₂UO₂Cl₄ melt) and processing of the data obtained in terms of the mass action law showed that the formation-decomposition reaction of the cation-cation complex can be described adequately only using the equation of reaction in the form NpO₂Cl ₄ ^3? + UO₂Cl ₄ ^2? ? {Cl₄ONpO?UO₂Cl₃}^4? = Cl^? (with the equilibrium constant of 1.3±0.1). Thus, the formation of the cationcation complex should be treated as replacement of chloride ion in the equatorial plane of uranyl(VI) by neptunyl(V), rather than as simple addition of UO ₂ ²⁺ to NpO ₂ ⁺ . The reverse reaction, decomposition of the cation-cation complex, consists essentially in replacement of neptunyl(V) by chloride ion.     

Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: I. Role of phase transitions in 5 M H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

Low-temperature oxidation of U(IV) with xenon difluoride, accompanied by formation of the UO ₂ ²⁺ ion in an electronically excited state, was examined in relation to the aggregation state and phase composition of 5 M aqueous H₂SO₄ solution. The reaction was found to proceed at a measurable rate in a supercooled liquid solution at T > 190 K and in a polycrystalline sample at T > 150 K. The chemiluminescence emitted by *(UO ₂ ²⁺ ) grows in intensity by several (up to 6) orders of magnitude during the exothermic phase transitions. The increase in the chemiluminescence intensity during phase transitions (crystallization of liquid, in particular, supercooled 5 M H₂SO₄ solutions) correlates with formation of the crystalline phase. Presumably, one of the factors accelerating the low-temperature reaction of U(IV) with XeF₂ during crystallization of the solution (along with concentration of the reactants in the intercrystallite spaces) is the catalytic activity exhibited by the freshly formed surface of the H₂SO₄ crystal hydrates.     

Liquid phosphors POCl3-ZrCl4-235UO 2 2+ -Nd3+

Liquid phosphors POCl₃-ZrCl₄-²³⁵UO ₂ ²⁺ and POCl₃-ZrCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ with concentrations of Nd³⁺ and UO ₂ ²⁺ of up to 0.75 and 0.12 M were prepared; the lifetime of the Nd³⁺ luminescence was up to 300 μs. The lifetime of the Nd³⁺ luminescence in POCl₃-ZrCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions decreases with increasing neodymium concentration, and this decrease is more pronounced than that in POCl₃-ZrCl₄-Nd³⁺ solutions. At molar ratio [ZrCl₄]/[Nd³⁺] < 1.5, the luminescence lifetime sharply decreases with decreasing ZrCl₄ concentration. The intensity of the absorption bands of the OH groups observed in the near-IR range of the absorption spectra of POCl₃-ZrCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions increases with increasing neodymium concentration. Upon storage of POCl₃-ZrCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions for 2 years without contact with the environment, the intensity of the IR absorption bends of the OH groups gradually increases, whereas the lifetime of the Nd³⁺ luminescence decreases to 60-80 μs.     

Synthesis and structure of crystals of UO<Subscript>2</Subscript>(C<Subscript>2</Subscript>H<Subscript>5</Subscript>COO)<Subscript>2</Subscript>·1.5L (L = methylurea or <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>’-dimethylurea)

The compounds UO₂(C₂H₅COO)₂·1.5L, where L is methylurea (Meur) or N,N’-dimethylurea (s-Dmur), were synthesized and studied by IR spectroscopy and single crystal X-ray diffraction. Irrespective of the kind of amide L, the structure of the compounds consists of two mononuclear uranium-containing complexes: cationic [UO₂(prop)L₃]⁺ and anionic [UO₂(prop)₃]^–, where prop is propionate ion. Both compounds belong to crystal-chemical group AB⁰¹M₃¹+ AB⁰¹M₃⁰¹ (A = UO₂²⁺, B⁰¹ = prop^–, M¹ = L) of uranyl complexes. The composition of 15 stable uranyl complexes that can be formed in the UO₂²⁺, B⁰¹–prop^––L–H₂O system was predicted on the basis of the characteristics of the Voronoi–Dirichlet polyhedra from the standpoint of the 18-electron rule.     

The calorimetry and thermodynamic functions of Nd Mg<Stack><Subscript>3</Subscript><Superscript>I</Superscript></Stack>Mn<Subscript>4</Subscript>O<Subscript>12</Subscript> (Me<Superscript>I</Superscript>-Li,Na, K) manganites in the range from 298.15 to 673 K

The ceramic technology is employed for synthesizing manganites of composition Nd Mg ₃ ^I Mg₃Mn₄O₁₂(Me^I-Li, Na, K). The X-ray technique is used to find that the compounds crystallize in tetragonal syngony. The parameters of their crystal lattices are determined. Their heat capacities are experimentally determined in the range from 298.15 to 673 K, which enables one to reveal second-order phase transitions. In view of these transitions, equations describing the C _p ^° ～ f(T) dependence are derived, and the thermodynamic functions C _p ^° (T), H°(T)-H°(298.15), S°(T), and Φ ^xx (T) are calculated.     

Oxidation of Np(V) with Xenon Trioxide in an HClO<Subscript>4</Subscript> Solution

Oxidation of Np(V) to Np(VI) with xenon trioxide in a 0.5–1.4 M HClO₄ solution was studied by spectrophotometry. The reaction rate is described by the equation–d[Np(V)]/dt = k[Np(V)][XeO₃], where k = 4.6 × 10^–3 L mol^–1 s^–1 in 1 M HClO₄ at 92°С. The activation energy is close to 92 kJ mol^–1. The activated complex is formed in contact of NpO ₂ ⁺ and ХеО₃ without participation of Н⁺ ions. The activated complex transforms into NpO ₂ ²⁺ and the products: ОН, Хе, and О₂. The ОН radical oxidizes Np(V). Admixtures of Со²⁺ and especially Fe³⁺ accelerate the Np(V) oxidation.     

Excitation of the uranyl ion in U(IV) oxidation with xenon difluoride in frozen aqueous sulfuric acid solutions: II. Effect of solvent and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> concentration

The effect of solvent (aqueous solutions of HClO₄ and H₂SO₄) on the low-temperature (T < 273 K) reaction of U(IV) with XeF₂, accompanied by formation of uranyl ion in the electronically excited state *(UO ₂ ²⁺ ), was examined. In the course of heating of 0.2–8.31 M HClO₄ solutions after their quick cooling to 77 K, the chemiluminescent reaction occurs at a detectable rate only at T > 220 K. In the similar experiments performed with 0.05–0.5 M H₂SO₄ as solvent, the emission appears at a considerably lower temperature, 165–175 K. The chemiluminescence (CL) intensity increases in the course of exothermic phase transitions occurring when the temperature of H₂SO₄ solutions (at [H₂SO₄] > 0.05 M) is changed. The increase in the CL intensity in the course of a phase transition is associated with the appearance in a multicomponent frozen system of a juvenile surface of H₂SO₄ crystal hydrates. The lack of CL at T < 220 K in HClO₄ solutions is associated with the fact that heating of the samples after their quick cooling to 77 K is not accompanied by formation of crystalline phases exhibiting catalytic properties toward the oxidation of U(IV) with XeF₂.     

Compounds WAl<Subscript>4</Subscript>, WAl<Subscript>3</Subscript>, W<Subscript>3</Subscript>Al<Subscript>7</Subscript>, and WAl<Subscript>2</Subscript> and Al-W-N combustion products

X-ray diffraction data are presented for combustion products in the Al-W-N system. New, nonequilibrium intermetallic compounds have been identified, their diffraction patterns have been indexed, and their unit-cell parameters have been determined. The phases α-and β-WAl₄ are shown to exist in three isomorphous forms, differing in unit-cell centering. The phases α′-, α″-, and α?-WAl₄ are monoclinic, with a ₀ = 5.272 Å, b ₀ = 17.770 Å, c ₀ = 5.218 Å, β = 100.10°; point groups C12/c1, A12/n1, I12/a1, respectively. The phases β′-, β″-, and β?-WAl₄ are monoclinic, with a ₀ = 5.465 Å, b ₀ = 12.814 Å, c ₀ = 5.428 Å, β = 105.92°; point groups A112/m, B112/m, I112/m, respectively. The compounds WAl₂ and W₃Al₇, identified each in two isomorphous forms, differ in cell metrics (doubling) but possess the same point group: P222. WAl ₂ ^′ : orthorhombic, a ₀ = 5.793 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. WAl ₂ ^″ : orthorhombic, a ₀ = 11.586 Å, b ₀ = 3.740 Å, c ₀ = 6.852 Å. W₃Al ₇ ^′ : orthorhombic, Pmm2, a ₀ = 6.225 Å, b ₀ = 4.806 Å, c ₀ = 4.437 Å. W₃Al ₇ ^″ : orthorhombic, Pmm2, a ₀ = 12.500 Å, b ₀ = 4.806 Å, c ₀ = 8.874 Å. The new phase WAl₃: triclinic, P1, a ₀ = 8.642 Å, b ₀ = 10.872 Å, c ₀ = 5.478 Å, α = 104.02°, β = 64.90°, γ = 107.15°.     

Liquid phosphors POCl3-TiCl4-235UO 2 2+ -Nd3+: 2. Nd3+ luminescence lifetime

Lifetime of the Nd³⁺ luminescence in POCl₃-TiCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions is almost independent of the concentrations of neodymium and uranyl ions; it attains the maximal value of 220 μs at [TiCl₄] = 0.20?0.35 M and increases with increasing synthesis time, reaching 220 μs at t = 40?60 min. Upon storage, the Nd³⁺ luminescence lifetime decreases by 20–30% in 2 months, and in 2 years it decreases to 60–80 μs in all the samples. Simultaneously with decreasing Nd³⁺ luminescence lifetime, multiple increase in the intensity of the absorption bands of the OH groups is observed in the near-IR range of the absorption spectra of POCl₃-TiCl₄-²³⁵UO ₂ ²⁺ -Nd³⁺ solutions.