Disclosed herein are methods for synthesizing N-(4-fluorobenzyl)-N-(1-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy)ph- enylmethyl)carbamide. Also disclosed herein is the hemi-tartrate salt of N-(4-fluorobenzyl)-N-(1-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy)-p- henylmethyl)carbamide and methods for obtaining the salt. Further disclosed are various crystalline forms of N-(4-fluorobenzyl)-N-(1-methylpiperidin-4-yl)-N'-(4-(2-methylpropyloxy)ph- enylmethyl)carbamide and its hemi-tartrate salt including various polymorphs and solvates.