A positive electrode active material for a secondary battery including: a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to a space group Fm-3m. The lithium metal composite oxide contains an element A.sup.1 of at least one selected from the group consisting of Ca, Al, and Si, and in the lithium metal composite oxide, a total of Ca, Al, and Si contents relative to a total of the lithimn metal composite oxide is 10 to 1000 ppm by mass.

A positive electrode active material for a secondary battery including: a lithium metal composite oxide having a crystal structure based on a rock salt structure belonging to a space group Fm-3m. The lithium metal composite oxide contains an element A.sup.1 of at least one selected from the group consisting of Ca, Al, and Si, and in the lithium metal composite oxide, a total of Ca, Al, and Si contents relative to a total of the lithimn metal composite oxide is 10 to 1000 ppm by mass.

A method of producing chlorine dioxide is disclosed. The method may include feeding a reaction mixture into a separator. The reaction mixture may follow a helical path through the separator and produce gaseous chlorine dioxide within the separator. Gaseous chlorine dioxide may be withdrawn from the separator and used to disinfect process water.

A coating material containing an oxyfluoride of yttrium and having a Fisher diameter of 1.0 to 10 μm and a tap density TD to apparent density AD ratio, TD/AD, of 1.6 to 3.5. The coating material preferably has a pore volume of pores with a diameter of 100 μm or smaller of 1.0 cm.sup.3/g or less as measured by mercury intrusion porosimetry. A coating containing an oxyfluoride of yttrium and having a Vickers hardness of 200 HV0.01 or higher. The coating preferably has a fracture toughness of 1.0×10.sup.2 Pa.Math.m.sup.1/2 or higher.

A coating material containing an oxyfluoride of yttrium and having a Fisher diameter of 1.0 to 10 μm and a tap density TD to apparent density AD ratio, TD/AD, of 1.6 to 3.5. The coating material preferably has a pore volume of pores with a diameter of 100 μm or smaller of 1.0 cm.sup.3/g or less as measured by mercury intrusion porosimetry. A coating containing an oxyfluoride of yttrium and having a Vickers hardness of 200 HV0.01 or higher. The coating preferably has a fracture toughness of 1.0×10.sup.2 Pa.Math.m.sup.1/2 or higher.

A luminophore having the empirical formula A.sub.3M*O.sub.xF.sub.9−2x:Mn.sup.4+ where A may be or include Li, Na, Rb, K, Cs, or combinations thereof. M* may be or include Cr, Mo, W, or combinations thereof. x may be or include 0<x<4.5.

A luminophore having the empirical formula A.sub.3M*O.sub.xF.sub.9−2x:Mn.sup.4+ where A may be or include Li, Na, Rb, K, Cs, or combinations thereof. M* may be or include Cr, Mo, W, or combinations thereof. x may be or include 0<x<4.5.

A coating material containing an oxyfluoride of yttrium and having a Fisher diameter of 1.0 to 10 μm and a tap density TD to apparent density AD ratio, TD/AD, of 1.6 to 3.5. The coating material preferably has a pore volume of pores with a diameter of 100 μm or smaller of 1.0 cm.sup.3/g or less as measured by mercury intrusion porosimetry. A coating containing an oxyfluoride of yttrium and having a Vickers hardness of 200 HV0.01 or higher. The coating preferably has a fracture toughness of 1.0×10.sup.2 Pa.Math.m.sup.1/2 or higher.

A coating material containing an oxyfluoride of yttrium and having a Fisher diameter of 1.0 to 10 μm and a tap density TD to apparent density AD ratio, TD/AD, of 1.6 to 3.5. The coating material preferably has a pore volume of pores with a diameter of 100 μm or smaller of 1.0 cm.sup.3/g or less as measured by mercury intrusion porosimetry. A coating containing an oxyfluoride of yttrium and having a Vickers hardness of 200 HV0.01 or higher. The coating preferably has a fracture toughness of 1.0×10.sup.2 Pa.Math.m.sup.1/2 or higher.

A battery includes a positive electrode containing a positive electrode active material, a negative electrode, and a solid electrolyte. The positive electrode active material contains a compound which has a crystal structure belonging to the space group FM3-M and which is represented by the following formula:

Li.sub.xMe.sub.yO.sub.F.sub.(1)

where Me is one or more selected from the group consisting of Mn, Co, Ni, Fe, Al, B, Ce, Si, Zr, Nb, Pr, Ti, W, Ge, Mo, Sn, Bi, Cu, Mg, Ca, Ba, Sr, Y, Zn, Ga, Er, La, Sm, Yb, V and Cr and the conditions 1.7x2.2, 0.8y1.3, 12.5, and 0.52 are satisfied.