In my MOF sample, I am substituting a larger metal atom (2+) with a smaller one (3+,4+) but the d spacing is increasing as per xrd and HRTEM. Reasons?

Samanyu Acharjya @Samanyu-Acharjya

16 November 2024 1 2K Report

What reasons could be possible as this is not according to the laws

James Henderson Mitchell

Answer to the Question:

Subject: Explanation for Increased d-Spacing in MOF with Smaller Metal Substitution

Dear Samanyu,

The increase in d-spacing in your MOF (Metal-Organic Framework) upon substituting a larger metal atom (2+) with a smaller one (3+/4+) might seem counterintuitive but can be explained by considering the following factors:

Coordination Geometry and Bonding: Smaller cations like 3+ or 4+ often have higher charge densities, leading to stronger interactions with the surrounding ligands. This can distort the framework geometry, resulting in an expanded unit cell and increased d-spacing.

Lattice Strain: Substitution of ions with different sizes can introduce strain into the lattice. The framework may relax in a way that increases the spacing between planes to accommodate the changes in bond angles or lengths.

Electron Density Redistribution: Higher oxidation states (3+/4+) can alter the electron density around the framework, causing changes in interatomic distances due to altered electrostatic and covalent interactions.

Solvent Inclusion or Interstitial Molecules: If smaller cations allow for easier incorporation of solvent molecules or interstitial species into the lattice, it could lead to an overall expansion of the framework.

Thermal or Defect Effects: The process of substitution might introduce defects or result in local changes in thermal expansion behavior, which could influence d-spacing.

A deeper investigation, perhaps combining computational modeling and complementary techniques like Raman spectroscopy or EXAFS, might provide more clarity.

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Best regards, James Henderson Mitchell CEO, MJ HSA Ltd

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