A main object of the present disclosure is to provide a solid electrolyte with high fluoride ion conductivity. The present disclosure achieves the object by providing a solid electrolyte to be used for a fluoride ion battery, the solid electrolyte comprising: a composition of Ce.sub.1-x-yLa.sub.xSr.sub.yF.sub.3-y, in which 0<x, 0<y, and 0<x+y<1; and a crystal phase that has a Tysonite structure.

Metal halide optical materials (e.g., scintillator materials or persistent phosphors) are described. More particularly, the optical materials include codoped perovskite-type halides, wherein the codoping ion is present at a molar ratio of 5000 parts per million (ppm) or less with respect to all cations. For example, the optical material can be a codoped trihalide having the formula ABX.sub.3 where A is one or more alkali metal, B is one or more alkali earth metal, and X is one or more halide that is doped with up to about 10 atomic percent of a dopant ion and codoped with up to about 5000 ppm of one or more isovalent or aliovalent codopant ion, such as a tetravalent ion (e.g., Zr.sup.4+), a trivalent ion (e.g., Sc.sup.3+, Y.sup.3+, Gd.sup.3+, or La.sup.3+ ion) or a divalent ion (e.g., Mg.sup.2+). The codoped material can have modified afterglow compared to a noncodoped material.

A method of fluorinating a solid compound of a rare earth metal to produce a fluorinated rare earth metal compound in solid form includes reacting, in a reaction zone, a solid compound of the rare earth metal and gaseous hydrofluoric acid, thus producing the fluorinated rare earth metal compound in solid form. The reaction takes place, in the reaction zone, in the presence of exogenous water, which is water that is exogenous to water that is produced in the reaction zone as water of reaction due to the reaction of the solid compound of the rare earth metal and the hydrofluoric acid. Conditions of temperature and pressure in the reaction zone avoid condensation of the exogenous water, the water of reaction when present, and the hydrofluoric acid.

A fluoride shuttle secondary battery includes a positive electrode layer, a negative electrode layer, and an electrolyte layer. The electrolyte layer is located between the positive electrode layer and the negative electrode layer. At least one layer selected from the group consisting of the positive electrode layer, the negative electrode layer, and the electrolyte layer includes lanthanum fluoride and strontium fluoride.

A solution phase synthesis process for preparing a rare earth perovskite, the process includes reacting an alkali metal material with a first surfactant ligand in the presence of a first solvent to obtain a first precursor complex solution; reacting a rare earth metal halide with a second surfactant ligand in the presence of a second solvent to obtain a second precursor complex solution; and reacting the first precursor complex solution with the second precursor complex solution in the presence of a third surfactant ligand and a third solvent to obtain the rare earth perovskite; wherein: the rare earth perovskite is in the form of nanocrystals; and the first solvent and third solvent comprise a non-coordinating solvent.

A solution phase synthesis process for preparing a rare earth perovskite, the process includes reacting an alkali metal material with a first surfactant ligand in the presence of a first solvent to obtain a first precursor complex solution; reacting a rare earth metal halide with a second surfactant ligand in the presence of a second solvent to obtain a second precursor complex solution; and reacting the first precursor complex solution with the second precursor complex solution in the presence of a third surfactant ligand and a third solvent to obtain the rare earth perovskite; wherein: the rare earth perovskite is in the form of nanocrystals; and the first solvent and third solvent comprise a non-coordinating solvent.

A battery according to the present disclosure includes a positive electrode, a negative electrode, and an electrolyte layer positioned between the positive electrode and the negative electrode. The positive electrode includes a positive electrode material. The positive electrode material includes a positive electrode active material and a first solid electrolyte material. The positive electrode active material includes Li.sub.xMn.sub.yO.sub.2, where 0?x??1.05 and 0.9?y?1.1 are satisfied. The negative electrode includes Bi as a main component of a negative electrode active material.

The composite active material according to one aspect of the present disclosure includes an active material including Li, Ti, and O and a first solid electrolyte. The active material is a porous material having a plurality of pores. The first solid electrolyte includes Li, M, and X. M is at least one selected from the group consisting of metal elements and metalloid elements belonging to the 5th or 6th period. X is at least one selected from the group consisting of F, Cl, Br, and I. At least a part of the first solid electrolyte is present inside the plurality of pores.

A battery according to the present disclosure includes: a positive electrode; a negative electrode; and an electrolyte layer positioned between the positive electrode and the negative electrode. The positive electrode includes a positive electrode material. The positive electrode material includes a positive electrode active material and a first solid electrolyte material. The positive electrode active material includes an oxide consisting of Li, Ni, Mn, and O. The first solid electrolyte material includes: Li; at least one selected from the group consisting of metalloid elements and metal elements except Li; and at least one selected from the group consisting of F, Cl, and Br. The negative electrode includes Bi as a main component of a negative electrode active material.

A battery according to the present disclosure includes: a positive electrode; a negative electrode; and an electrolyte layer positioned between the positive electrode and the negative electrode. The positive electrode includes a positive electrode material. The positive electrode material includes a positive electrode active material and a first solid electrolyte material. The positive electrode active material includes an oxide consisting of Li, Ni, Mn, and O. The first solid electrolyte material includes: Li; at least one selected from the group consisting of metalloid elements and metal elements except Li; and at least one selected from the group consisting of F, Cl, and Br. The negative electrode includes Bi as a main component of a negative electrode active material.