tension
Words that end in -tion usually come from Latin, while words that end in -sion usually come from Latin or French. There isn't a strict rule, but -tion is more common and originates from Latin verbs ending in -tio, while -sion more often comes from Latin or French words ending in -sio or -sion.
Fusion Fission
I only know one word that starts with de- and endsin -sion is decision.
Expand\expandsion
I think the answer you want is "nouns" . tion and sion are suffixes that change a verb into a noun. eg. associate -> association . The trick is to know which ending to use.
Some nouns that end with the suffix -sion are:abrasionconclusionconfusionderissiondiffusiondivisionemissionfissionfusionlesionmansionmissionomissionpassionpermissionremissionsessionvision
Sion Mills railway station ended in 1965.
Roman Catholic Diocese of Sion ended in 1628.
Words ending with "ition":AbolitionAcquisitionAdditionAmbitionAmmunitionApparitionAuditionCoalitionCompetitionCompositionConditionContritionDefinitionDemolitionDispositionEditionExhibitionExpeditionExtraditionIgnitionImpositionInhibitionInquisitionIntuitionMalnutritionOppositionPartitionPetitionPositionPrepositionProhibitionPropositionRecognitionRenditionRepetitionRequisitionSuperstitionTraditionTransitionTuitionWords ending with "sion":AbrasionAdmissionAggressionAllusionAscensionCessionCollisionCommissionCompassionCompressionCompulsionConcessionConclusionConcussionConfessionConfusionConversionConvulsionCorrosionDecisionDiffusionDimensionDiscussionDiversionDivisionEnvisionExclusionExcursionExplosionExpressionIllusionImplosionIncisionInvasionMansionObsessionSeclusionSessionTensionVision
tension and lesion.
sionist, sionism
Some examples of words with the suffix -sion are discussion, decision, and expression.