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It should make sense that phosphorus has the positive charge,and gold itself should have the negative charge in gold phosphide(and for any other gold phosphides) because phosphorus has a lower electronegativity than gold (phosphorus is 2.19, gold is 2.54),but then, they can both just share electrons. Is this true or not?

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Because their electronegativity difference is less than 0.4, I think the bond would have a lot of covalent character. I personally don't think it is a good idea to treat this substance as an ionic compound and assign a positively charged ion and a negatively charged.

According to a chemical supply company, $\ce{Au}$ is +3 and $\ce{P}$ is -3. We typically write the cation first, so from the formula $\ce{AuP}$, you should get another clue that the gold ion is positively charged (sort of if you have to assign a positively charged ion).

I don't think you can assign charge signs solely on electronegativities. Atomic radius and position on the periodic table also play roles in determining which atom is more positively charged.

For electronegativity, more reference here.

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The electronegativity of an atom can change in different environments. Gold as an isolated atom is less electronegative than fluorine, but when oxidized to the $+5$ oxidation state it can force a positive charge on a difluorine ligand. Phosphorus is apparently more electronegative than silicon, yet it becomes positively charged (by donating its extra valence electron to the band structure) when used as an $n$-dopant in silicon-based semiconductors.

So we cannot tell a priori how the charge will be distributed in a material with predominant covalent or metallic bonding from the electronegativities seen in an element chart. Unless we have detailed calculations to resolve this, the actual distribution of charges in gold containing phosphorus may be hypothesized as practically anything.

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