Defamation is not applicable in acquittal of 498A cases. The reason is that the prosecution simply failed to prosecute the accused with strong evidences and the process throughout the trial or investigation is the due process of law which cannot be resembled with defamation! Though after acquittal you can go for filing of divorce on the ground of cruelty in dragging the accused in false 498A case.
In the eyes of law, both husband and wife are one person and the communication of a defamatory matter from the husband to the wife or vice versa is no publication and will not come within the purview of section 499 of IPC.
Acquittal in a498A case is a valid ground for seeking divorce. As regards to defamamtion, any suggestion depends upon the nature and substance of allegation.
In addition, there is the option to invoke Section 208 IPC and also, an action for malicious prosecution.