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I have read that $\ce{LiHCO3}$ does not exist in solid state. Is it because of the size of anion-cation concept or is it something else? So why is it only for lithium and not for sodium too?

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    $\begingroup$ This site states that it is a solid at room temperature, guidechem.com/dictionary/5006-97-3.html but information on this compound seems scarce. $\endgroup$ – porphyrin Feb 27 '17 at 11:58
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    $\begingroup$ Why does beryllium peroxide not exist? Why is magnesium peroxide only metastable but by the time you get to barium the peroxide is fully stable? Polarization by small, densely packed cations tends to pull polyatomic anions like peroxide or bicarbonate apart, leaving smaller nations in the compound like oxide or carbonate. $\endgroup$ – Oscar Lanzi May 1 '17 at 1:27
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    $\begingroup$ CRC handbook of chemistry and physics (also latest 97th Ed.) includes $\ce{LiHCO3}$ in the table "Physical Constants of Inorganic Compounds" (4-70) as white powder, slightly soluble in water 0_O. $\endgroup$ – andselisk Aug 29 '17 at 2:52
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Claiming that a particular chemical compound "doesn't exist" is difficult to prove. The only convincing proof would be to show that neither LiHCO3 nor any of its hydrates can be isolated from (aqueous) solution. Essentially, you would have to show that using any of an almost infinite number of methods, that it couldn't be prepared at any temperature or pressure - not very likely to have ever been done, imho. So, I'll ask you for your source for such a claim, or better yet, I'll ask you to check and verify that your source - which you accept as being accurate - has made stringent efforts that demonstrate the claim as well as having published that work in peer reviewed literature. As an aside, there are plenty of compounds which require "waters of hydration" in their crystal structure. If it really doesn't exist, then the reason is simple: kinetically the reaction 2LiHCO3 → Li2CO3 + H2O + CO2 is spontaneous and energetically favored. It just seems to me that if I placed Li2CO3 in a pressure cell with water and CO2 at a billion gigapascals of pressure, I could make the bicarbonate salt, but perhaps some other structure is more likely...and anyway, generally what they mean by "doesn't exist" is that it "isn't known to exist at standard temperature and pressure".

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It's is because $\ce{Li}$ is in diagonal relationship with $\ce{Mg}$ in periodic table. As magnesium hydrogen carbonate is not obtained in solid form, $\ce{LiHCO3}$ also cannot be since both have similar properties.

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due to small size of lithium its hydration enthalpy is very high and moreover hydration enthalpy is higher than lattice enthalpy of lithium bicarbonate

hence lithium bicarbonate exists in solution state only

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Due to the small size of lithium, it will polarize the large anion oxygen, the $\ce{C-O}$ bond will weaken and $\ce{Li-O}$ bond will strengthen, they will split, and they will not be in the solid state.

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