Table 4 Thermoanalysis data of heteropoly compounds containing transition metal

    The thermostability of all of transition metal salts is greater than their homologus acids and increases with the cation charge increasing. The thermostability of bivalent transition metal salts also changes with cation electronegativity. The lower the electronegativity is, the greater the thermostability is.
2.1.6 The rare-earth metals cation
Rare-earth metals have empty 5d orbit, so they can become the position to transferring electron for catalysis. When led-in the counter position the rare-earth metal can change obviously the catalytic feature of HPA. Table 5 shows the exothermic peaks data of HPA salts containing rare-earth metals on the DTA curve [8]. The thermostability of HPA is related to not only composition and structure, but also the radius of counter cations. The thermostability of the same series HPA salts will become greater as the atomic number of rare-earth counter cations increases.
    According to the experiment result from table 5, the thermostability order is: LnHGeW₁₂O₄₀＞H₄GeW₁₂O₄₀, LnHSiW₁₂O₄₀＞H₄SiW₁₂O₄₀, and LnHSiW₁₂O₄₀＞LnHGeW₁₂O₄₀.
    The thermostability order of HPA salts containing rare-earth metals is the same with that of HPA, which indicates that it is the heteropoly anion affecting the thermostability of LnHSiW₁₂O₄₀ and LnHGeW₁₂O₄₀.

Table 5 Exothermic peak temperature of rare-earth metal on DTA curve

(H^* is the one replacing of Ln)

2.2  The internal heteroatom
2.2.1 The internal transition metals heteroatom
People studied the thermostability of HPA containing different internal ligand atoms by experiments including TG-DTA, XPS, IR and solubility experiment.
Table 6 shows the DTA data of HPA containing transition metals^[9,10].

We can obtain the rule from the data of table 6 and 7:
(I): The thermostability of the same series of rare-earth ternary HPA salts is similar.
(II): The thermostability of the same series of ZMW₁₁ is different with the difference of Z. The thermostability of MW₁₁ containing Fe³⁺，Al³⁺ and Ga³⁺ is greater than that containing Co²⁺，Ni²⁺ and Zn²⁺. So the thermostability of MW₁₁ becomes greater with increasing of electron change of Z.
(III): The thermostability of both MW₁₁ and Ln(MW₁₁)₂ is greater than that of corresponding Keggin structural MW₁₂O₄₀.

2.2.2 The internal nonmetals heteroatom
The thermal decomposition of the heteropolytungstates [PW₁₁M(H₂O)O₃₉]^{n- (}M=Mn, Co, Ni, Cu, Fe) has been studied by Cavaleiro, and the decomposition products identified by powder X-ray diffraction, FTIR and NMR spectroscopy ^[11].
    Seven new-type pentabasic heteropoly complexes with the general molecular formula K₁₀H₅[Ln(PMo₅W₄V₂O₃₉)₂]·nH₂O (Ln = La, Ce, Pr, Nd, Eu, Gd, Dy) synthesized by Zhou and the thermal stability studied by water solubility test, TG-DTA, XRD and IR at various temperature ^[12,13].
    According to Table 8, the main factor influencing thermostability of Ln(XM₁₁)₂ and Ln(X₂M₁₇)₂ is the thermostability of ligands XM₁₁ and X₂M₁₇, the subsequence is the same with that of the corresponding XW₁₂O₄₀ and X₂M₁₈O₆₂^[14,15].

Table 8 Thermoanalysis data of different internal nonmetal atoms