Synthesis and X‐ray Crystal Structures of the New, Three‐Dimensional Ethylenediphosphonates [A{HO
3
P(CH
2
)
2
PO
3
H
2
}] (A = Alkali Metal, Tl, or NH
4
): Solid Brönsted Acids That Exhibit Topotactic Intercalation and Deintercalation of Ammonia
Synthesis and X‐ray Crystal Structures of the New, Three‐Dimensional Ethylenediphosphonates [A{HO
3
P(CH
2
)
2
PO
3
H
2
}] (A = Alkali Metal, Tl, or NH
4
): Solid Brönsted Acids That Exhibit Topotactic Intercalation and Deintercalation of Ammonia